What you probably witnessed was a dual drive multi chain-ring hybrid drive race bike with a helmholz motor. Very astute of you to pick it out by the chainring size alone. Most people don't catch that but you may have also noticed a whirring noise as she pass you. This is the motor kicking in. People generally use batteries disguised as water bottles and have a button on the stem which turns it on and off. Very crafty use of new technology. You were doing great to keep up as long as you could up against that machine. I'll be she didn't even look like she was working too hard.