Someone wants more than he or she can handle, as in He's thinking of acquiring a third business, but we think his eyes are bigger than his stomach. This expression alludes to someone taking more food than he or she can digest. [Late 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.