XL motor and M motor on IR receiver = same output?
It seems like you're mixing up gear ratio with torque with power. It sounds like you have the following scenario:
 - same gear ratio
 - same overall vehicle
 - two different motors
 - (almost) same final speed  

And you're asking why the different motors don't change the speed much. Here's what I know:  


The vehicle's mass and internal friction place a load on the motor. This is approximately constant for your vehicle, because practically the difference in mass of the M and XL motors is negligible compared to the total load anyway.  
The M motor has a higher unladen speed but a lower overall torque than the XL motor. This means that for the same load, the M motor's speed will be more greatly affected than the XL motor.  
So in this case: your vehicle might drop the XL motor from 200 to 150 RPM, but drops the M motor from 600 to 175 RPM, because the M motor is affected more. Coincidentally this particular load drops both motors to almost the same final RPM.  
If your vehicle were even more massive, the XL motor's speed would drop, but the M would drop even further, quickly becoming slower than the XL. Add enough weight and eventually that M will drop to 0 RPM while the XL will still be chugging along.