If we were applying reality to 40k models, then all of the "walkers" would probably not be able to walk. Their legs and hydraulic mechanisms would not support their massive structures. It is all about imagination.
Meh, if they were made of materials known to us at the moment.
If they were made of a stronger material...or a lighter material or some lesser combination of both? Possible. Maybe reinforced by some process unknown to us.
Go back 3000 years (or heck, *300* years) and say we could build 120+ story buildings and they'd laugh at you. The means and materials just weren't available back then*.
*Okay, maybe they were? I don't know, I'm no pre-industrial revolution historian. Still, the point stands. With sufficient technology, anything is possible. We know that the Imperium has access to some fantastically durable, malleable metals or alloys (thanks to the likes of Terminator armor), so is it so far-fetched they'd apply them to their walkers and titans?