Forget that, the Leman Russ MBT is a DAoT tractor that they strapped guns to. Only Baneblades are really tanks, and they were light tanks. Humanity used to be OP as fuck.
Speaking of being OP as fuck - The Emperor Titan isin't even the biggest or most powerful of the bunch. Castigator-class autonomous bipedal weapons platform was the "original titan" and was much more advanced and bigger than your regular ones. Also it had head over it's shoulders, so that's something. Sadly, as usually, it was the Chaos who got the best toys.
And speaking of tanks and Mechanicus - a lot of people don't understand what big impact Horus Heresy had on them, because Mechanicus weren't as prominient as Space Marines and most other imperial forces aren't a thing yet so in the end it usually melts down to "spess mareens shoteeng eech oteer". Pior to HH, Mechanicus lost a lot, but they still had knowledge to field things such as Krios and Triaros tanks which are literally one of most advanced things that ever existed. Those don't even have any meaningful armour, because interlocking energy fields (which proably means they're something else than Warp shields, due to the fact that interlocking Warp shields makes them weaker) made it nearly invincible. Don't even get me started on Volkite weaponry and Taghmata Omnissiah which were literally removed from the existence due to sheer power of the weapons they used against each other.
waitwaitwait LEMAN RUSSES ARE TRACTORS
Yes.
They're armored tractors. And to that, before this they used an actual tank, Malcador, but ultimately decided that the tractor is better.
Imperium has a long history of this, and Death Korps of Krieg were experts on that - Ragnarok,
Bruennhilde (that's the actual tractor) and Siegfried to name just some. Honourable mention to Blitzen and Götterdämmerung, but those are cannons made out of scrap, not pure tractors.