In low floor vehicles traction equipments pertrude into the saloon, which are bigger than stepwells.
This may have been true of earlier designs and some at the 'cheaper' end of the market (CAF Urbos) still lose seats of the motor bogies, but most other vendors have solved this with current designs.
The 1997 built Sydney Variotram is a true 100% low floor. There are no 'intrusions' into the cabin for 'traction'. There are seats over the motor boxes. There is some space lost to equipment - there is a 19" track behind the driver at each end. That equipment on later models is on the roof with everything else.
The Citadis 305s for Sydney have a step to seats over the motor bogies - but that isn't for the motor - it's for the suspension springs
There are no other 'intrusions' into the cabin, they don't lose 4 seats per motor bogie like the Combino or Urbos cars do.
But even on a 'compromised' design like the Urbos 100 or the Combino, the loss factor due to equipment protruding into the cab would be way less than that lost to stairwells - at least not if the car had a decent number of doors. A single ender with only 2 doors MIGHT be able to make that claim, but a modern multi-door artic - nope.
More space can be lost to doing things like specifying the wrong doors (Ansaldobreda Sirio with bus style doors that swing into the car, I do not know what they are called, only that riding a Napoli Siro you careful where you stood near the doors or you would get hit by them as they opened!)