IIRC, doesn't the Monster Manual say that size increases from more HD represent animals born to be larger than others, so a smaller animal won't ever grow to be larger through advancement? Instead, the "advancement" is what you do before the PCs even see the monster.
Anyway, if the whole point of this was to "smite" people who think Fighters easily keep up with Druids, it's a lost cause since that argument was mostly resolved a long time ago. It's like coming to a science forum to smite people who think space is full of aether.
And frankly, even the critter in the OP isn't better than a well optimized beatstick anyway (sure, it hits hard and is good at tripping, but if you wanted to outdamage a well optimized beatstick you'd want a Fleshraker with Shock Trooper). I think a strong charger (Lion Barbarian 1/whatever else as long as it has Shock Trooper, Leap Attack, and full BAB) would out damage it, and possibly so would a well made archer. Not to mention "beatstick" now includes the ToB classes. The Druid himself of course brings a lot more to the table.
JaronK