Lemons huge & Lemons reg sz on same tree

A student of mine is doing an science fair experiment and has a lemon tree with ridiculously huge lemons on it as well as regular size ones! He wants to test the lemons and find out what is causing this difference. He wants to know what the huge lemons have that the regular ones don't. please help!!!!

Lucia

 

Check the branches, you are probably seeing the rootstock producing fruit as well as the scion.