Well, it is a bit of a drip feed!
It happens every time?
So why did your husband express surprise?
Presumably you like them enough to have them round, you know it's their way, I wouldn't be bothered.
In their minds, perhaps their contribution is "dessert for the evening" so anything left over wasn't their contribution to the evening. I think you're more likely to over cater a cake than a main course.
Don't get me wrong, I wouldn't do it. But I don't see it as a big deal.
You go to each other's for dinner regularly - so I don't think the "gift" for the host element of it fits, exactly. A first time dinner with someone I would take wine and flowers. But this sounds a much less formal affair.
Next time, say "that was delicious - can we split it actually - I'd love another slice tomorrow".
The only time I'd have a problem, is if they took their extra cake home, but got the arse if you tried to!
Perhaps because you never do, they think you're not consumers of leftover pudding, and genuinely think it's going to get binned? 