If there's a visible lump after feeding, and he's acting hungry the next day, I think you may be mis-interpreting how he's acting. If a feeding leaves a visible lump, he should hide for a few days while digesting.

What do you have him in? Temps? Hides? All that stuff...

And there's no need to waste the rats. You can feed multiple prey items per feeding, or just shorten up the time between feedings...i.e...feed every 4-5 days insted of every 7 days...