Vessels are able to have one more prey than their limit if they eat a vessel who already has her own prey, and the three lamia (snake tail) characters are each worth one more capacity point since they're big like that.
Example: If your vessel has 4/5 prey, and you command her to eat a vessel who isn't a lamia and has one prey already inside her, she will end up with a 6/5 tummy if the attack is successful. Recycle this example but your target is instead a lamia, and it will still be true only if your attacking vessel has 3/5 prey instead, because again lamias are one extra capacity point because of their largeness.
This'll happen in the lobby at times because your vessels will automatically feed themselves with vessels you never possessed while you're on the battlefield. In fact if you go to the part of the lobby where you feed your vessels, you can see which exact vessels she gobbled down while you were away!
I think this is one of those things that's easier done than said, so let me know if you still don't quite get it!