Kumin,
I'm not sure about any relation with stem thickness, but I agree that there seems to be some correlation with large leaf size and large fruit size.
Look at these citrus and loquats plants from my garden.
1.) Peluche is the largest know loquat fruit (according to members)
2.) 9 pounder lemon, one of the largest lemon (my grafted branch is only 2yrs old so had three fruit =2 lbs ea, but large leaf).
3.) Roy's seedling pomelo- it has the largest leaf (6" x 13-1/2" see photo), and fruit is pretty decent size (2.8 lbs).
4.) Seedling loquat, unknown variety but seems to have the largest leaf so far in my 30+ varieties. Hope the fruit gets large.
Roy Pomelo leave size = 6" x 13-1/2"