In the original version of Star Wars (or Episode IV), we saw banthas on Tatooine, but nothing larger. Then in Return of the Jedi, we saw the sarlacc (which, to be fair, isn't native to Tatooine). In later versions, we saw a number ofdewbacks in Mos Eisley. (I've also read about the krayt dragon, which is even larger -- we do see a skeleton of one in Episode IV, but no living one or any indication how old the skeleton is.)
The reason this started to bother me is because the dewbacks reminded me of dinosaurs and I remember reading about how larger dinosaurs were vegetarians because there just were not enough animals that could live in their range that they could eat to get the energy they need.
We see almost nothing growing on Tatooine. Luke's uncle is a farmer, but a moisture farmer, not a food or plant farmer, and there's no other evidence (at least in the movies) of many plants growing on Tatooine.
And on Hoth, Han comments, "There isn't enough life on this ice cube to fill a space cruiser." Again, there's almost no evidence of plant life on Hoth, but the Rebels are using tauntauns (that are native and likely from near where they are) and Luke is captured by a wampa, which is also kind of large.
On both Tatooine and Hoth, we see these animals in areas of little or no vegetation, so unless they all store food for extreme periods of time (to allow for long migrations), they're in or near their natural habitat.
On the other hand, on Dagobah, where there is plant life in abundance, and more than enough to support larger animals, we get only one hint of a big animal (when Artoo is presumably eaten up and spit out under water). (At least on Naboo, there's an abundance of plant life and large animals.)
Are these animals too big to realistically survive on ice and desert planets where there is so little plant life? Is there anything to give us reason to believe there is enough life at the lower end of the food chain, with plants (or maybe some kind of algae or bacteria) that can support such large life forms on these planets?