[quote name='Sinistar']Gah this is frustrating.. someone needs to make a video tutorial on how much you need to eat to get this tutorial. XD[/QUOTE]
I managed to get that trophy yesterday. Here's my findings:
Stretch out nice and long (at least the length of the map), and start eating. Once you've eaten enough small stuff, you can eat bigger stuff (I had some roosters on my map I couldn't eat right away, but could after filling about halfway up.) Once you can't eat any more without losing an object out your backend, shrink up with L3 and ONLY L3- do not use R3!
Once you've shrunk (and all the stuff you ate finishes blasting out), you'll find 2 things have happened: 1) your butt's a little bigger. 2) You haven't lost the ability to eat larger objects. Repeat the process, but this time, skip the small stuff and go straight to the largest things you can eat, moving up into ever-larger things. You may have to switch maps to find bigger things to eat. As you do this though, your butt will gradually get larger and eventually the trophy will trigger. (At which point you can press R3 and shrink your butt back to normal.) I never got big enough to eat houses- I think I was up to small trees/bushes or so when it finally kicked in.