co2. If you are blasting them with co2 they will throw out a lot more leaf to deleaf vs not running co2. Only thing I can think of if you are on the same nutes. The only other thing would be more N on your side if using tap vs his unless you are on the same water supply.
Have you ran CO2 at blasting levels? I just ask because I run CO2 and I have also not ran CO2 and leaf production is not affected by higher co2 levels.