I think it's the Courier New font and how special characters like the colon and semicolon appear at large font sizes. Those two characters are always a little darker/thicker in this font, but your eye doesn't pick that up as easily until the size is around
14 pt and then they start to standout.
As for the size of the space marker, that always picks up its formatting from the previous character so in your example, only when the "s" in the word This is bolded does the space after become larger.
Kind Regards, Rich ... http://greatcirclelearning.com