Yes, different systems might have different size of reserved memory. But this is just a rough estimation anyway, different kernel, drivers, glibc, X, Qt,... might also have different memory requirements. It also depends how big the additional repositories are, etc... So many factors, that's why YaST just warns "it might crash". It's up to the user to decide, and if it does not work then just restart the installer and try it again without the online repositories. That's all what we can do.