not trying to jump on anyone but Abi you must be honest..the health and safety of your animals is why we all try to help then get a bit miffed when things seem to not add up
as in your cat "kept trying to chase them"..then "he's too fat to chase them"??
If you are honest about everything you will sometimes get people disagreeing with you but at least most of us will also keep offering advice and help to you
so.....
Please don't think your cat won't hurt or even kill them because he might..even by accident.and even while you are there as they can move quickly
...and personally I wouldn't leave two such young baby bunnies to free range unsupervised, too many places for them to get into, under and stuck..or things to nibble which could cause blockages..or they could be sat behind a door that someone opens quickly and result in injury or death.
I'd keep them caged unless you are with them until they have grown a bit and built up their confidence.
so try to get hold of a larger cage as that blue one is much too tiny, see if anyone near you would lend you one perhaps