I don't really have any idea's for what you could do for this case, but I wouldn't be surprised if something existed. I'm just thinking in my head a little about how the browser probably works.
I'll give an example. Think about this page. Imagine we wanted to animate your avatar to be 3 times it's size. So every 15ms or so, the width and height get increased. Every time you change the size, the browser needs to recalc other other things on the page that depend on it. Because once the image gets too big, it starts stretching that table cell it's in, and when the cell changes, it will affect the dimensions of the cell to the right, which in turn affects all the stuff in that etc... The dimensions and positions of all these things need to get constantly recalculated and redrawn.
Now, if your image was taken out of the flow of the document, for example via position:absolute, changing the size wouldn't change anything else. Might look a little funny when it seems to start growing over the page, but I think the browser is likely to do this far more efficiently.
I know yours doesn't change the size, I just though it to be a good example.