As a breeder I know that size doesn't matter, once a 3lbs wild buck knocked up my 10lb doe - there might be more extrem pairings, but I'm pretty sure rabbits are very good at that stuff.
About him not finishing - I recently saw a Brexit protest sign that said "Pulling out never works", true for humans, so I wouldn't bet that rabbits are safe from that.
Fastest accidential breeding that I witnessed was done in way less than a minute. When they are in the mood they are fast, but granted, that was a very experienced doe with a long history of surprise litters - I still have no clue whatsoever how she managed it last year.
Rabbits only don't breed like Rabbits when you want them to breed, that's really true
, but leave a gate open for just the blink of the eye and the deed is done.
So, everything is possible, but better be prepared anyway - it isn't so much work and you'll sleep better.
My bet too would be that she's not pregnant, but I've seen too much to take any risks at that.