Toxic Wasteling will grow in size if allowed to eat a critter. An easy way to test this is to go the the "Ninja Turtle" spot in the Dalaran Sewers and just stand by as Toxic Wasteling happily pounces on all of them one by one, dissolving their crittery flesh and growing in size. This appears to cap out between 4 or 5 critter-meals, though there is no physical buff or debuff to show stacks.
The Wasteling will also eat a Death Knight's bloodworms.
Patch 3.3.0 (08-Dec-2009): added