(1) Limited gene pool for chocs. Folks who want an all choc litter have a real problem because the available animals for breeding may not be the best.
(2) Field trial folks breeding for the big dogs. I don't think it's necessary to get a good field trial dog, it's just one of those rumors that gets started. (Just for contrast's sake, the "rumor" among Golden Retriever breeders is that the "little red dogs" make the best field trial dogs.)
My choc is a beautiful, neat little girl who is actually 1/2" under the AKC standard height . . . but when we measured her in AKC she roached her back up a little . . . so she's officially 21 1/2" tall in AKC, but in USDAA she's 19 3/4" tall . . . luckily! (USDAA height class changes at 20 inches).
She is an odd breeding (I call her my illegitimate child) because her sire is a Show or Conformation Lab while her mother is all field trial breeding. She got the small size and short-coupled build from her dad, and the slenderness and drive from her mom. Her mom's dad (NFC AFC Storm's Riptide Star) is the only choc ever to win a national field trial title, and he is a big rangy dog. Her dad is a champion who throws all three colors, and her litter was about half and half black and chocolate (IIRC, 5 black and 4 choc).
She certainly is not ugly, although she's a little too light in build behind for a conformation dog and her ears are a little too big (a trait thrown by her maternal grandsire along with a funny little bump on her nose).
Shelley (sorry I don't have a pic of her "stacked" - I'm at work).
Shelley's dad, Can. CH Sumo Simbra Black Bordeaux - your typical small solid show dog
Shelley's maternal grandsire - your typical big rangy choc. What's funny is that his pedigree is solid black for 5 generations back, he's just an outcrop from a "nick" - i.e. a couple of hidden genes on different branches of the pedigree that just happened to pop up chocolate.
In this case, the back breeding to a conformation type produced a dog with better type but still retaining the field ability. Good compromise IMHO - she also won't eat you out of house and home (45 pounds).