Read this : https://www.fourwindsgrowers.com/other-growing-questions/largeoversized-leaves.html

I have 6 other lemon trees placed at the same window with plenty of sunlight. They all have normal sized leaves, except for this one. So the explanation in the link doesn't apply in the circumstances.

One thing you should do (I hate to say it), but you need to trim that tree. Cut off the central leader because otherwise it will not branch. I think the consensus is to cut it to about 30 inches?? I don't remember the exact number. In your case, since it is an indoor tree, I would cut it about as high as you want it to be now.

Sorry...just re-read your post. Only 7"..don't trim yet!

Laura is correct. Cut the leader at 28 - 30 inches, and the tree will branch out. Seedling lemon trees are especially up right growing, more so than many other varieties - Silica

Did you ever find out why you had the huge leaves? I have a like tree with leaves at the end of one branch that are triple the size of the rest. For some reason I can’t read your post or some of your comments. Thanks.

Nope. One other lemon tree started growing giant leaves.

Light levels dictate leaf size, and sometimes some plants have more "juice" (energy) to feed growing leaves if it's timed right---it's all about flushes. The plant has to decide to either grow more leaves or grow bigger leaves, and sometimes larger leaves win out in low-light conditions. Plants capitalized "ROI" - return on investment - long before people wore business suits and worried about the bottom line!

Blue light has the tendency to create very large leaves.