I have noticed that when I bought my Serissa a few
months ago, it had large leaves, whereas now the new
leaves have approx. halved in size.

That is what you want, the smaller, the better. Probably you are giving it
better light than it had before. Keep doing what you are doing.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)