I can think of a few reasons, depending on how early you got the BKS:
- it has extremely high base damage but not great scaling, so if you get it early enough it wrecks pretty much everything you come across.
- most "normal" enemies in Dark Souls are, all things considered, pretty frail. Even the mini-boss type things only tend to have maybe 600 or so hp until much later on in the game.
- comparatively, anything above fodder/mook-level enemies in Dark Souls 2 tends to have at least 1k hp and a lot of them have fairly high resistances to various kinds of damage as well.
Basically DS2 has much more robust enemies in general. I bet the Mastodon would seem much more impressive if you were using it in DS1. In fact, let's do a quick comparison...
+10 Mastodon GS in DS2: 300 base AR, A/E str/dex scaling.
+15 Zweihander in DS1: 325 base AR, C/D str/dex scaling.
So in DS1, the Mastodon would've been better (with high strength for scaling bonuses) than Zweihander, albeit only by a little, putting it on par with ultra greatswords. The DS1 Claymore, by comparison, was only 257 base AR with C/C scaling at +15, so its DS2 version is actually buffed a bit (as is the DS2 Zwei, which now has it at 350 base with B/C scaling).
So yeah, the weapons are still as strong as ever. It's the enemies that are tougher to kill that make them seem weaker than DS1 weapons.