It's possible. One of the optimizations in UIDrawCall makes it use the full buffer rather than the trimmed version of it as trimming a buffer means garbage collection allocation. Put a Debug.Log inside UIDrawCall around line 432 to see how big of a buffer is being set.
Doing some math it doesn't quite add up though. Maximum possible mesh size is 65,535 vertices, 12 bytes for position, 8 for texture coordinates, 4 for color, that's 1.6 megabytes. Even with maximum number of indices (65k * 1.5 = 98.3k), at 4 bytes each, that's 393 kb, so absolute total would be under 2 megs for the lagest possible mesh.