While this is just as much of an educated guess as all of the other answers, I would suspect that part of it is probably due to congestion at one or both terminals at the scheduled times of day. You can see this effect even on domestic flights. Even for the same cities on the same airline with the exact same model of aircraft, it's not unusual for scheduled durations even on domestic routes to vary by 5-15 minutes depending on time of day, especially at major hubs (which both Boston and CDG are.)
I do suspect that aircraft type also has something to do with it in this particular case, though. As other answers have mentioned, the 757 cruises a bit slower than the 777 or 747 (at 35,000 ft: Mach 0.80 for 757 vs. Mach 0.84 for 777 or 747-400.)
Edit: My answer originally stated that the 757 was limited to 120 minute ETOPS (e.g. that it was required to stay within 120 minutes of a potential diversionary field.) However, according to Boeing, 757-200s (both RR and P&W engine configurations) were upgraded to 180 Minute ETOPS certification. I have updated the map below from gcmap to show areas where the 757 is allowed to fly under 180-minute ETOPS, which includes the entire North Atlantic. As such, the 757-200 is not limited on where it can fly between BOS and CDG by ETOPS regulations, so ETOPS shouldn't be a factor in why the Delta flight takes longer.