For Mathematica, Rubi and Maple, the buildin system function LeafSize is used to find the leaf size of each antiderivative.
The other CAS systems (SageMath and Sympy) do not have special buildin function for this purpose at this time. Therefore the leaf size for Fricas and Sympy and Giac antiderivatives is determined using the following function, thanks to user slelievre at https://ask.sagemath.org/question/57123/could-we-have-a-leaf_count-function-in-base-sagemath/
For Sympy, which is called directly from Python, the following code is used to obtain the leafsize of its result