Such small animal companions.

[Vamptastic]

So, I've noticed that all of the Familiars/Animal Companions are typically smaller versions of their wild counterparts, if their counterparts are anything bigger than medium or small.

Like for example, a regular T-Rex. I forget how big it is, Large? Huge? Something like that. But if you get one as an animal companion, it's either small or medium.

Why? I mean, yeah, I know there would be disadvantages to having a Large creature(walking in doors or in dungeons, etc), but I'd be willing to pay for that in exchange for having a giant T-Rex following me around. So how come we can't have really big animals as companions?

[Lincoln Hills]

I think it was simply a mechanics-balancing issue: Huge and larger creatures are not only harder to fit into typical adventuring areas, they're a bit shaky in terms of balance issues. Their size and strength can provide CMBs that are virtually unstoppable against normal-sized foes, and most of 'em have natural armor and attack routines sufficient to knock the whole party up one APL. In short - they can get too good.

Me, I'm waiting for the stats for a hamster companion. Go for the eyes, Boo! Go for the eyes!

[Vamptastic]

Vorpal Adamantine hamster ball.

[Craig Mercer]

Some are, some aren't.
Take a closer look at some animal companions, they start out smaller than their wild counterparts but then get to their regular size.
And Wolf start out Med (size of a wild wolf) but goes to large (dire wolf size).

[SteelDraco]

They apparently decided they'd rather have PCs with animal companions start with powered-down versions of just about any animal, rather than the old level adjustment system that powerful animal companions used. It does allow you to start play with the companion you want, though it's kinda awful if you think about it too hard (you're sending baby creatures in to fight to the death!)

It also restricts some concepts, and I'm not too sold on the balance for a lot of the animals - big cats and some of the dinosaurs are clearly better than the vast majority of the available options. I'd also like to see a few more animals that start as Large so you can have a mounted druid effectively that isn't riding a horse or a camel, or just walking around next to their animal companion until 7th level.

[Vamptastic]

I just want my T-Rex, man.

[Lincoln Hills]

I particularly like the implication it seems to create when you compare it with the fluff: that your critter suffered from stunted growth all its life because your bond with nature was just too weak to allow it to grow to the size of the same beasts in the wild. Your love is stunting its growth (that poor little guy!): all its litter-mates that you didn't love as much grew to normal size. Worse, any other animal you later choose to replace it with always comes from the same vitamin-deficient end of the gene pool.

[Sean K Reynolds]

Or your animal companion was the runt of the litter/last survivor of a predator's attack and was starving to death/pygmy throwback, and wouldn't have survived without your intervention.

In my experience, the runt or sickly animal in the litter tends to be a very affectionate and loyal pet—perhaps because you took care of it when its mother and siblings didn't.

[Lincoln Hills]

Or that, yes. I'm ribbing you guys, Sean. I already explained that I know why you guys went with this option; it just looks funny to see all those Pokemon-sized allosaurs and gorillas out there.

[RJGrady]

It's not too hard to mix-and-match if you want something a little different. The horse can be any large, fast animal you want it to be; just reskin its primary attack as one of the attacks of your target creature.

[Sean K Reynolds]

Or use any animal companion's stats as a flying butt.

[Lincoln Hills]

(Having consulted the link):

Just when you thought gamers - and the Internet - couldn't get any classier...