First, we need to determine what kind of zombie we're working with: 'live' or 'dead.'
Live zombies? Almost impossible. Not only do they lack the advantages of an undead (tireless, relentless, needs no food or water), but why on Earth would they link together or otherwise combine to make a bigger monster? What advantage would be conferred? Sure, with enough creativity, you could handwave zombie parasites, through chemical signals, communicating the need for 'something bigger' and making individual zombies weave their bodies together into one big zombie (or something similar) but...
Common sense dooms the idea. A tank-sized zombie, formed of human bodies, would likely collapse under its own weight. Instead of creating an effective vector/combatant, all that would result is a disgusting mass of gore. In my mind, I don't believe live zombies can do this unless...
With some sort of genetically engineered parasite or zombie, who knows? It's possible that a live zombie could either integrate cells from its devoured victims (sort of like a leaf sheep or blue glaucus) and mutate them into 'zombie' cells, therefore growing in size over time, making the big ones the smarter, more capable zombies of the bunch. This would mean that zombies would grow bigger and bigger over time, until they either die (if the zombie's too big, it could collapse or become unable to move or support itself via its vital organs) or reach some sort of biological cap on size. These zombies could possibly become tank-size, depending upon whether they can adapt for larger size, which depends on your zombie vector.
'Dead' zombies are dead bodies given a semblance of life through unnatural forces, like dark magic, demons, or evil spirits. For dead zombies, it make sense that 'destroyed' zombies could mass together into some sort of conglomerate monster, driven by resentment and malice, creating something like the Gravemind in Halo.
If you don't like that idea, maybe use a pseudo-science explanation or scientific magic. Magic in the Sword of Shannara series or ghosts and related supernatural phenomena would be the thing to research then. You might find some inspiration from that, you might not, but you never know!
Game Tactics: Please note, I may use this idea myself. Did you know cells can live outside the human body? Like, human cells? Thus, it's possible that as zombies are injured or destroyed in combat, their lost mass (ie. lost and infected cells) could mass together into their own organism!
If a parasite is the cause of your zombies, all sorts of monstrosities could result, or else gene transfer (which can and does occur between species) could be the cause. Slimes made out of green zombie flesh would be the most likely result, but what about chimeras? Rats, roaches, or even lizards eating creeping masses of gore could develop into horrific mishmashes of human and animal, the infected cells fusing with the native cells like they did for this pig.
So, hope it helps! (And that you don't have nightmares...)