The city of Dallas has been called “Big D” since at least the 1930s. Denton is a nearby city that has been nicknamed “Little D” since at least 1950.
   
“Denton, Tex., Jan. 16—‘Little D’ is losing out to ‘Big D’ in numbers of students at North Texas State College” was printed in the Waxahachie )TX) Daily Light on January 16, 1950. “‘Little D’ is serving notice to all of Texas that as an automobile sales center it’s read to take on ‘Big D’ or any other metropolitan city at any time, on any car, or any deal” was printed in the Denton (TX) Record-Chronicle on February 22, 1950. “And speaking of size, the nicknames have given residents countless opportunities to make the occasional dick joke. Chances are, you giggled upon reading ‘Big D’ and ‘Little D’ just now” was printed in the Dallas (TX) Observer on September 9, 2021.
 
Another Denton nickname is “Little Austin.”
   
   
Wikipedia: Denton, Texas
Denton is a city in and the county seat of Denton County, Texas, United States. With a population of 139,869 as of 2020,[9] it is the 27th-most populous city in Texas, the 197th-most populous city in the United States, and the 12th-most populous city in the Dallas–Fort Worth metroplex.
(...)
Nicknames: Little Austin, Little D, Redbud Capital of Texas
       
Newspapers.com
16 January 1950, Waxahachie )TX) Daily Light, pg. 4, col. 1:
Dallas Passes
Denton In Number
Of NTSC Students
Denton, Tex., Jan. 16—“Little D” is losing out to “Big D” in numbers of students at North Texas State College.
 
Newspapers.com
22 February 1950, Denton (TX) Record-Chronicle, pg. 1, col. 2:
‘BIG D’ CLAIMS SAID ‘HOGWASH’
Denton Auto Dealers Issue
State-Wide Car Challenge
“Little D” is serving notice to all of Texas that as an automobile sales center it’s read to take on “Big D” or any other metropolitan city at any time, on any car, or any deal.
 
Newspapers.com
15 July 1956, Denton (TX) Record-Chronicle, “Denton Would Cooperate With Dallas If Unhurried ” (editorial), pg. 4, col. 1:
So if Denton is forced to take some action, we hope the farmers and ranchers won’t be too harsh on “Little D.”
   
Newspapers.com
22 September 1957, Denton (TX) Record-Chronicle,  “High School News” by David Robinson, sec. 2, pg. 6, col. 4:
Anybody who wishes to come is cordially invited to attend the “Little D Jamboree” on the parking lot of the First Baptist Church.
 
Newspapers.com
21 May 1958, Denton (TX) Record-Chronicle, pg. 2, col. 6 ad:
FLASH! Little D ahead of Big D on the
Premiere Showing of This Big One!
(The movie God’s Little Acre.—ed.)
   
Newspapers.com
18 January 1959, Denton (TX) Record-Chronicle, “Best of Sports Events Are Yours for Asking,” sec. 5, pg. 4, cols. 2-3:
But in spite of the fact that Cowtown and Big D are conveniently located, Dentonites can find just about any sport they want right here in Little D.
   
Newspapers.com
22 February 1960, Denton (TX) Record-Chronicle, pg. 4, col. 2:
YESTERYEAR
Looking Back Through the Record-Chronicle Files
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DENTON CAR MEN
OUT TO SELL UNITS
FEB. 22, 1950
“Little D” is serving notice to all of Texas that as an automobile sales center it’s read to take on “Big D” or any other metropolitan city at any time, on any car, or any deal.
 
Newspapers.com
13 August 1965, Denton (TX) Record-Chronicle, “Views” by Bill Rives, pg. 1, col. 1:
Si Ragsdale Jr., the bouncing ball of energy who masquerades as manager of the Chamber of Commerce, has composed a song about his favorite city, Denton.
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Because there are no dents in Denton, no dents in Denton…No dents in little D…There is TWU and North Texas State, too…But there are no dents in little D…(It’s the top of the Golden T)...
 
Newspapers.com
28 August 1966, Denton (TX) Record-Chronicle, “What To Do In Denton? There’s Bowling, Golf…,” sec. 8, pg. 9, col. 1:
“Little D”—that’s Denton in case you didn’t know—can take care of your needs.
   
Newspapers.com
9 September 1974,  Denton (TX) Record-Chronicle, “Mean Green plans surprises for SMU” by Mack Owens, pg. 1B, col. 2:
It’s a promoter’s dream come true—that little Saturday night shindig down in Irving. Hayden Fry, coming back to haunt his old school; Little “D” (Denton) against Big “D” (Dallas); a fired head coach facing his forced assistants etc.
 
Newspapers.com
16 September 1995, Covallis (OR) Gazette-Times, OSU Game Day sec., pg. 1, cols. 3-4:
Williams is homeward bound
He’ll play ‘D’ in ‘Little D’ near his home in ‘Big D’
By Brook Hatch
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Williams has been looking forward to today for several months, ever since he discovered the Beavers would be heading to Big D (Dallas) for their game up the road at Little D (Denton).
 
NBC News
Behind The Badge
A Texas police officer’s wife is killed in what the cop claims is an accident. Seven years later, the case seems like it will never be solved — until a complete stranger, a reporter, takes interest in finding justice.
Jan. 29, 2010, 7:45 PM EST / Source: Dateline NBC
By Lee Cowan
In the lore of the Lone Star state, Texas lawmen rank pretty high: the good guys in the white hats. Their badge brought prestige - their gunfights, the history books. But in the modern-day town of Denton, Texas—the “little D” outside “the Big D” of Dallas - there served one Texas lawman with a curious past, all locked up in a box.
       
Twitter
chloë
@chloewithdots
dallas is the big d, denton is the medium d, duncanville is the little d
8:14 AM · Jul 29, 2021 from Duncanville, TX·Twitter for iPhone
 
Twitter
Sam Melnick
@sam_bamalam
Replying to @SchumakerA
Not to be confused with “Big D” Dallas, TX and “Little D” Denton, TX.
12:22 AM · Aug 21, 2021·Twitter for iPhone
 
Twitter
MF
@melamama23
Replying to @starsmoonandsun
Let me know when you want to visit Denton (little d) welcome to Texas! I hope you aren’t disappointed lol
5:19 PM · Aug 29, 2021·Twitter for iPhone
 
Dallas (TX) Observer
The 5 Best North Texas City Nicknames (and Some of the Worst)
MARK E. MOON SEPTEMBER 9, 2021 4:08AM
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Big D (Dallas) / Little D (Denton)
These nicknames may apply to different cities, but they convey a similar theme that hits on the same beat. As the names imply, the cities are relatively different in size.

And speaking of size, the nicknames have given residents countless opportunities to make the occasional dick joke. Chances are, you giggled upon reading “Big D” and “Little D” just now.