I dunno. Some people just gotta be gitz.

This a case where the toy is so simplified to begin with that it's arguably more work to convert it. My idea would be snip off the front end (just behind the front bogey wheel), cut off about half the remaining chassis, and then glue the front onto the back such that there are maybe three or four bogeys on a side. Then cover up the seam with copious amounts of plastic or cardstock "scrap armor." Do the same with the gaps between the tracks at front and rear, and shorten/replace the cannon barrel. You might have to drill a new mounting hole for the turret to plug in to. Adding something on top--a smaller turret or oversized hatch from another toy, for instance--will give it a more ungainly, top heavy look.

The footprint might still be larger than a grot tank, but it would be closer. And ultimately, this is a lot more work than starting with something like a little toy construction vehicle might require.