How can sion and galio have a rivalry?

The unstoppable force meets the immovable object

: "Broken stone, broken bones!"

I mean come on, this is Sion we're talking about! No matter how big Galio is, Sion will take him on with abnormal strength.

Sion doesn't stop and is really strong.

Sounds like a pretty good reason for Sion to harbor colossal resentment towards him.

Sion is capable of dealing with his size.

He feeds off of conflict, and can become infinitely more powerful. In the lore it takes a whole battalion to hold him down. His strength makes up for his size, (even though he is like 8 feet tall) and he can probably at the very least knock him over if he rams him at full speed.