The tubes should be put into the tyres, and the tyres fitted back on to the rim, before the tyre is inflated.
You can put enough air in beforehand that the tube is round rather than flat, but that's all, and even then you may have to let some air out in order to get the second side of the tyre back on the rim.
Tyres will get enormous if inflated outside the tyre. It has been a party game at cycling club annual dinners - everyone puts a pumpful of air in, then passes it on, and the person who makes the tube explode wins the booby prize. How big depend on how even the thickness of the tube rubber is, but you can be talking 15 feet round and 6" thick.