“Don’t worry about biting off more than you can chew. Your mouth is probably a whole lot bigger ‘n you think” is sometimes described as a Texas proverb or an old-time cowboy proverb, but there is no evidence that this was used in the old west. The saying first appeared in Don’t Squat with Your Spurs On!: A Cowboy’s Guide to Life (1992), by Texas Bix Bender.
   
   
Google Books
Don’t Squat with Your Spurs On!:
A Cowboy’s Guide to Life
By Texas Bix Bender
Layton, UT: Gibbs Smith
1992
Pg. 34:
Don’t worry about biting off more than you can chew. Your mouth is probably a whole lot bigger ‘n you think.
 
Twitter
Mike Miller‏
@LibertarianMike  
Don’t worry about biting off more than you can chew. Your mouth is probably a whole lot bigger than you think. #quote
12:34 PM - 31 Dec 10
     
Bryan Patterson’s Faithworks
Posted on June 20, 2013
Cowboy Wisdom
(..)
12. Don’t worry about biting off more than you can chew. Your mouth is probably a whole lot bigger than you think.
 
Twitter
A True Cowboy
‏@ATrue_Cowboy  
Don’t worry about biting off more than you can chew. Your mouth is probably a whole lot bigger than you think.
#CowboyWisdom
9:57 PM - 20 Oct 12
 
Twitter
CommonManTwit‏
@CommonManTwit  
A mouthful!: There is a Texas proverb which says, “Don’t worry about biting off more than you can chew.  Your ... http://bit.ly/14HYVyD
8:29 AM - 24 May 13