The log was far larger than any rock we have seen Cera break... and Spike manages to crush a rock back in movie three, after all.
Breaking that rock didn't really tell us anything about how strong Spike is; it gives us an idea of how HEAVY he is!
On a side note, it also makes you wonder what it must have been like for Petrie to be sat on by Spike in movie VI...twice.
You also have to consider just how small some of these rocks are... Cera, especially in the first film, is not really that big. She rarely, if ever breaks anything larger than her, whereas the log Spike moved, while not as heavy as a rock per unit of volume, was almost certainly far heavier overall. We also see Cera is unable to move a similar (but larger) piece of wood in movie nine.
Even a small rock isn't an easy thing to break, and most of the ones we see Cera shattering are at least the size of her own head. Admittedly, the rocks in LBT seem to break more easily than real-world stone (otherwise one would have expected the original Sharptooth and Chomper's mother to have suffered brain damage
), and I can't remember any examples of Spike attempting to smash rocks with his head, so we can’t really compare Cera’s strength to Spike’s in this regard anyway. However, in LBT IV, Cera managed to knock most of the leaves off a tree in a single charge (even the adult spiketail seen attempting the same in LBT VIII during Mr. Thicknose’s lecture wasn’t anywhere near as successful), whereas in movie V Spike had to ram a tree multiple times to dislodge a single treestar (which may have ultimately been accomplished by Petrie’s simultaneous attempts to pluck it himself.).
Also, if you're talking about the log that she was helping her father move, that was a freshly fallen TREE; very different from a hollow log. The tree also still had all its branches on; something like that doesn't roll very easily, and is even harder to push.
