The bite evo already lists how size scales- 1d6 medium, 1d8 large, 2d6 huge.
You have a medium eidolon, and you used two different methods of increasing the effective size. With the evo, it acts like it is large (1d8) and with the feat it acts like it is huge (2d6).
While I am partially inclined to say that your stat multiplier on your bite is x2, it is entirely possible, that, by raw, it was made x1.5, and then made x1.5 again.
As in putting more points into your bite evo on a single attack build might be useless- it possibly only works on multiple attack builds (where the bite would get x1.5 on both str and power attack, making it fairly valuable for non-pouncers and attacks on opportunity).
I would look to the rules threads for these kinds of questions first.
For build concerns... I would personally do this kind of build with a biped using a slam. The advantage there is that bipeds have more reach as you go up in size, which means you can do an AoO build. A nice fortuitous AoMF and the reach evo, and you could get a ton of AoOs. And with lunge on top of that, and you won't even bother worrying about the quardrupeds' pounce.
Oh, and remember- you get a single iterative with your natural attack at level 9 due to the multiattack ability. At least as long as you only have 1-2 attacks. This was a feature added to animal companions to balance wolves (who only have a bite) with creatures like the various cats (who have 3 attacks), and this carried over to eidolons. You still get x1.5 on both hits (I had to do a lot of digging through Paizo's NPCs to find a high level druid with such an animal, but yes, that is how the devs play it at least) so that is a pure upgrade for you. So with this one big hit build, you kinda hit like a great sword user, so that is cool.