Likely, leaf drop was that the tree was going dormant for winter. If you are in cold climate, the tree will loose most, if not all of it's leaves. If you are in a mild climate some leaves will remain on the tree. Typically, you will not see new leaf growth until the spring.

Leaf drop during Spring, Summer, Autumn is probably due to lack of water.

Large leaves are probably because the tree is maturing.

As mylife said, elms drop leaves as winter approaches. If kept inside it may hang on to some leaves. My guess is that it is over-watered. During winter it needs to be kept just damp. They do not consume much water unless they start growing again.

March/April is about the time elms will start growing again. Do not worry about the big leaves now. Move it to a warm well lit southern window (not on the sill - it can get cold next to the glass). Let it grow. Start giving it some dilute fertilizer when you water but check the soil first. The top inch or so needs to dry out. Then drench it water. You can probably go a week without watering again depending on how fast it is growing.

mylifeeclectic very valid point, Even if elms are in a mild climate they usually react to the Winter and try to force themselves into at least a partial dormancy. It's for the best, if the tree didn't go dormant it would get exhausted... as it is not meant for continual year round growth like say evergreen varieties.

I would stay away from any liquid based fertilizer. However if you have a soft slow release fertilizer for bonsai add some of the little pellets to the top. I would start to address the big leaves ASAP. Elms tend to grow long shoots of leaves. Proper foliage and branch structure is maintained by cutting these shoots down to 2-3 sets of leaves. this will allow for a more full canopy and stimulate growth. It also might be time for a re potting right now in early spring. I have care guides and tutorials on how to re pot Japanese Elms (which are very similar in care to Chinese Elm)

Japanese Elm Care Guide

I keep my Chinese Elm outside, 24//7, 365/365. It never comes in.

It's a tree. Other than any plant that needs a tropical environment, and the usual list of houseplants (specifically bred and grown for indoor conditions) trees should stay outside. Most Chinese Elms can withstand temperatures down to -14 t- -16 degrees C. the only thing likely to kill it is frost after rain, so keep it sheltered awy from rainfall, and water s appropriate.

My tree is in full leaf, healthy and growing successfully. You can tell from where I am, that weather conditions are... changeable, to say the least....

These pictures were taken in June....