I did play around with that a bit. I had the impression that toBlob worked for larger sizes than toDataURL, and that larger sizes are possible in FireFox than in Chrome (in FireFox, I created 10000 pixel wide images…). But no conclusion about the reason yet.
The following is a sandcastle that contains three “Capture” buttons, with scaling factors 1.0, 3.0, and 8.0 (and it seems to consistently fail with 8.0, because toBlob returns “empty” data), just for the case that someone wants to try it out: