Can I make a cable core bigger to use a bigger cable lug?

In case you badly need it and have no other choice, yes.
If the lug is too big for the cable- take a part of c cable that is almost the same length as the barrel. Put this part inside the barrel together with the cable core that you need to crimp, so that all the space inside the barrel is filled, then do the crimping. The result will be reliable as maximal loads are calculated for cable diameters less than the nominal size of the lug.