Thanks for that! I guess I now understand why I was unable to send a screen video a while back regardless of the fact that it appeared to be appropriately sized.
Edit: I cut it down to less than the size my ISP said was acceptable, but that heretofore unknown 30% overhead factor apparently stymied me.
The ratio of output bytes to input bytes is 4:3 (33% overhead)
This is because email is meant to send text, not raw data, and does not support ascii lower than 32 or higher than 127, whereas raw consists of values 0 through 255. The encoding narrows the variation by expanding the size.