There was an idea discussed in a previous thread which I thought was pretty interesting. Basically, mechs as we know them aren't actually designed to be mechs, but are actually the power armor for enormous species which are much larger than humans. Humans turn this power armor into a mech by inserting a cockpit for themselves and altering the power-armor to move through the usage of controls at the helm of this cockpit.

This handles a few things nicely. First, it explains why mechs exist in the first place (they might not be intuitively better than a tank, but it's cheaper to convert an already existing power-armor into a mech than it is to build a tank from scratch). They allow humans to interact with the giants on their own level. And it allows you to explain differing forms for mechs by having different varieties of giant species. A humanoid species of giant elves for example makes for a convenient humanoid mech, but a crablike giant species allows you to make more of a crabwalker mech for example.