Admissions offices in the pre-Internet era used to send acceptance letters in the mail. College seniors (or others applying to educational institutions) checked the mail each day, hoping for the best.
 
An applicant’s hope was to receive a “fat letter” and not a “thin letter.” A thin letter usually signified a one-page rejection letter from the educational institution. A fat letter, however, was often an acceptance letter accompanied by many other forms, such as dorm information, food plans, health services and other essential information for new students.
 
“A fat letter means acceptance; it contains all kinds of forms to be filled out” was cited in print in 1962. “In the particular parlance and mythology of college-bound high school seniors, ‘fat letters’ are acceptance notices from colleges” was cited in 1969.
 
 
5 August 1962, Dallas (TX) Morning News, sec. 1, pg. 17, col. 4 ad:
National Observer Tells You
Why Top Colleges
Reject 2 out of 3
“If the letter is fat—wild joy. A fat letter means acceptance; it contains all kinds of forms to be filled out. But a majority of student receive thin rejection letters, and must turn to other, less-known colleges.
 
4 March 1966, Kingsport (TN) Times, “The Big Sale (Or How Not To Pick A College)” by Debbie Mueller, pg. 20, col. 7:
However, generally speaking, college-bound seniors are still waiting for a fat letter from the college of their choice. A thin letter of acceptance would be OK, but usually a thin letter is a bearer of bad news—short and to the point. While a fatter letter has literally bundles of joy, and of which must be signed and checked “yes"ed and “no"ed and “maybe"ed for pages ad infinitum.
 
23 April 1969, Rockford (IL) Morning Star, “‘Fat Letters’ Are Talk of Spring at High Schools” by Linda Mathews (LA Times-Wash. Post Service), pg. B20, col. 3:
LOS ANGELES—If you don’t know what “fat letters” are—as opposed to “thin letters”—the chances are that you’re not 17 years old, or the mother, father, sister, brother, teacher, guidance counsellor or passing acquaintance of a 17-year-old.
 
In the particular parlance and mythology of college-bound high school seniors, “fat letters” are acceptance notices from colleges. They are fat because besides the “yes, we want you” message from the dean of admissions, they also contain catalogs, housing brochures and medical forms; all the paraphernalia for the freshman year.
 
‘Thin letters,” on the other hand, usually contain only a single sheet of letterhead stationery on which is written a tactful, soothing, but, nonetheless, emphatic rejection.
 
Google Books
Find a Stranger, Say Goodbye
By Lois Lowry
Boston, MA: Houghton Mifflin
1978
Pg. 1:
“IF YOU GET a fat letter, it means you’ve been | accepted. If you get a thin letter, forget it.”
 
Becky Margeson made the pronouncement. No one was really listening; they’d all heard it before. It was part of the college-admissions folklore that circulated every spring.
 
19 April 1988, State-Times (Baton Rouge, LA), “Cost of college education rivals mortgage” by Joan Beck (KNT news service), pg. 7-B, col. 1:
CHICAGO—Those fat letters of acceptance that successful college applicants will be getting this month and after may hold good news for them. But they may leave parents in a state of acute financial shock.
 
New York (NY) Times—Motherlode log
What Should Vassar College Do About Its Admissions Mistake?
By JUDY BOLTON-FASMAN JANUARY 30, 2012 5:08 PM January 30, 2012 5:08 pm
Electronic college admissions decisions make me nostalgic for the old slim letter/fat letter days. Waiting for a piece of paper in a stamped envelope was a rite of passage. Plus, sending word the old-fashioned way provided more opportunities to catch an error like the one that happened at Vassar College over the weekend.
 
Twitter
Chris Adamson
‏@invalidname
@LiuwdJelly Oh, right. College admissions are online now. God I’m old. Do you guys even get to stress about “fat letter” / “skinny letter”?
9:28 PM - 31 Oct 2014