the rod size does not affect the pushing power, only the main bore size. That looks OK for a good splitter.

A bigger rod has the advantage that the retraction is faster though.

My guess is the wedge is dull, and/or the pump is bad or just not sized properly. You could try to operate the splitter off a tractor remote, if it works then, it's the pump.

+1 that the wedge is on the wrong side.

Edit, a real bad internal leak in the cylinder could also be a problem.

#2 Edit, tried to correct the vocabulary mistake, thanks Thud.