Why are Adobe Illustrator "high quality" PDFs larger than "press quality" PDFs?
I just created a test file with a only simple 6 letter word set in Myriad regular. And when diffing the two PDFs, the color profile embedding area of the High Quality Print PDF was the only place there was significantly more information stored, about 500kb more in fact. in a 650kb file. There was still a 4kb discrepancy when matching the color profiles settings from Press and High Quality Print, so there are likely some differences in how the fonts are embedded as well.
The bottom line seems to be that Press quality assumes that you will be printing on a 4 color press (CMYK Colorspace), and modifies the color information in a potentially lossy way.