The indoor atmosphere has a lot of stimuli that makes it seem like you are hitting bigger serves than perhaps you are. The sound of the racket striking the ball is somewhat echoed, whereas outside it isn't as impressive. Additionally, during warmups indoor serves hit the court then bounce into the back curtain. You are able to get a sense of serve quality by the pop the ball makes on the curtain as well as how high up on the curtain your balls are hitting.
The fence outdoors just doesn't provide the same feedback. Then of course there are the elements outside which have already been addressed.