I have a C&C cage that is 2x3 cubes. I can't give you the exact dimensions as I am absolutely terrible with numbers, but I have a hut, a CSW, food dishes, toilet paper tube, snuggle sack, tshirt, and a small stuffed hedgie in there and Diggory still has plenty of room to roam around.
A lot of hedgies look very round but are not overweight. If your hedgie is chubby he will have extra fat rolls around his legs and have trouble balling up. Hedgehogs come in all different shapes and sizes so even weighing him isn't a good way to tell if he is overweight or not. If you don't see fat rolls and he can ball up, he is most likely normal. Also a lot of people on here suggest free feeding which is what you say the owners before you were doing with the hedgie. That is normal and shouldn't really make him overweight unless he has a bit of an eating problem or is on food with too high of fat content. Start counting the kibble or weighing it each night to see how much he eats, the experts on here can tell you if it sounds about normal or not.
As for being standoffish, you may have just woken him up in the middle of a good sleep and he was grumpy about it. A lot of people play with their hedgies around 8 or 9 at night when they've had time to sleep all day and should be well rested and happy.
It sounds like you are doing the bonding things right and all I can really suggest is sitting with him under a blanket or in a snuggle sack on your lap and letting him just get used to you. Even if he is sleeping it counts as bonding. I wouldn't suggest hand feeding him though, that can cause him to begin to associate your fingers with food and nip at them whenever he wants a treat.
most people feed the mealworms with tweezers or chopsticks. I am not squeamish with worms though so I just pick them out and set them on the floor in front of my hedgie, he never gets one straight from my hand.