Look at these pictures:

This isn't a tree of mine. These pictures have been posted on a Lithuanian forum in the citrus topic, along with a bragging of how "g r e a t l y" their little Calamondin tree was growing.
Personally I don't see anything great in this. Iron chlorosis is obvious. But why are the leaves so big? Is this caused by using too much of unbalanced fert? Did the tree get too much Nitrogen and too little micronutrients?
How can this be avoided?

Both too much nitrogen and low light can cause this. Plants grown indoors often grow larger leaves with a longer internode spacing to compensate for less light.

The production of oversized leaves is a fairly common trate for young citrus trees. This occurs with many varieties of citrus, and other types of fruit trees. As the tree grows it will produce leaves standard to its variety. Occasionally the same type of leaf production occurs after pruning. - Millet (898-)