Look, dude was mad smart. They called him the professor. A shinobi's strength isn't only in their technique,or muscles, or how much chakra they have. Their are a lot of things that can make a shinobi strong. Hashirama can create a forest in an instant, but maybe Hiruzen was so good with kage buunshin that he was able to get the jump on anybody. Maybe his deduction skills were so good that he could break down his enemies and their techniques in an instant. Knowing their weakness and exploiting it.
See the thing people are forgetting is that the Sarutobi clan does have a special skill in a way. They seem to catch on to things and learn techniques very fast. They were skilled enough to maintain an alliance with the akimichi, nara, and Yamanaka(?) clans for at least a few generations. Long enough to have a tradition with them, and to have these clans almost as guardians and vice versa. In fact, Sasuke was named after Hiruzen's father. A man so revered that people are naming their children after him.
In power I'm sure, in the end, Hashirama would be stronger in jutsu strength and size, and most likely in stamina. However, it seems they are portraying Hiruzen as extremely intelligent, and from his battle, he seems to have jutsus strong enough, which, with the right opportunity, can give him the victory.
He was considered the best of the five Kages of his generation, and we have seen almost all of them in action. They are all extremely powerful. So give credit where credit is due.