A leg of lamb contains a lot of meat as well as a large bone. The author wanted to establish that the entire leg of lamb was consumed in order for the evidence to be completely and not just partially destroyed. That was why Roald Dahl decided to have so many policemen investigating the murder. He made it plausible that many men would be in the household because the victim had been a policeman himself, so they would be strongly motivated to catch the killer. Dahl needed to have them spend a long time on the premises to give the leg of lamb time to be properly cooked in the oven.
They spend several hours searching for the murder weapon. The author specifies that it is nearly nine o'clock when Mary invites the investigators to eat the fully cooked lamb. They are all big, active men who probably haven't had anything to eat since lunchtime.
There was a good deal of hesitating among the four policemen, but they were clearly hungry, and in the end they were persuaded to go into the kitchen and help themselves.
So there are four hungry men cutting up a fully cooked leg of lamb which probably didn't weigh more than about four or five pounds. They could easily devour all the meat and completely destroy the potentially incriminating evidence.
One of them says, with regard to the murder weapon:
"Personally, I think it's right here on the premises. Probably right under our very noses."