Citrus Leafminer. The damage is done, but you can treat now to prevent further damage. Bayer Advanced Fruit & Vegetable Insect Control (Imidacloprid) can tree this systemically, but you really do need to apply about 2 weeks before you see CLM in your area. You don't mention where you're from in your "Zone" section, but here for me, in S. California, I need to apply in June, at least by mid-June, to give the product enough time to travel from the roots to the newest growth (which is what CLM likes to leave it's eggs under, so that the larvae can hatch and easily tunnel into the tender leaves). I only need to treat once a year, those in other areas of the county need to treat twice, once right before first flush in early spring, and then again, in June/July. Or, you can spray with a combination of Spinosad and a hort oil (like Neem or Volck), starting about a week or so before CLM hits your area, and continue to spray every 3 weeks through the season (again, not sure what that is for you, for me, through October.) Just don't use a hort oil if temps threaten to go above 85 degrees, or you'll fry your leaves. Also, be sure to spray in the evening, after the bees have gone back the their hives, and try to avoid blossoms if you can. Patty S. Here is a link that might be useful: UC IPM: Citrus Leafminer

It is normal for citrus to drop older leaves, especially on established trees like this one. And it's always a good idea to fertilize regularly. But on the third leaf from the right, I think I see Whitefly larvae and their white stuff they leave around them. And I think the first two leaves may have a little bit of sooty mold, which would come from the Whitefly larvae droppings, a sweet sticky substance. It looks like a pretty mild case. On a tree that size they won't really damage it, it's more of a nuisance then. But if I were you I'd take the hose with a spray end and give it a good hard wash with water, especially on the under sides of the leaves. That will get rid of a large percent of them, then I'd do that every week for a while, say a month or so. That will help get rid of new ones as eggs left behind, hatch out. Overall though, johnmerr and pip are right, it's in pretty good shape, just need some food.

Yes, the pot looks small and the plant looks underfed. You are giving it only about a quarter of its nutrient needs: Citrus Focus instructions say "Add 10ml of Citrus Focus in every litre of clean water, weekly. If leaves are pale or yellow, increase frequency of application."

pip313: These haven't been fertilized. Fertilizer damage would have been more apparent when they were repotted in the new soil (if there was higher level of fertilizer with Miracle Gro) and wouldn't appear several months after being repotted. Vladimir: I haven't applied any fertilizer. I've only repotted them into the 3 gallon containers with miracle gro. OJ: Well, I am pretty further north so we get short days during the winter. This specific tree is genetically predisposed to produce giant leaves. The nodes are extended on the right branch that started growing less than a year ago. The main branch on the left is fine in terms of node spacing.