a volume pedal in general, other then provide a handsfree control of adjusting either volume or gain(depending on placement in effect chain), should not increase volume or gain in the way of boosting them beyond its own amount
Why?
A volume pedal is a passive device, no circuit to boost anything, its role is just to reduce whatever signal that goes into the input.
If "x" is the amount of signal strength, putting a volume pedal after it, is not going to give "x + __" increase in volume or gain.
What the volume pedal does instead is, reducing from what you have initially, aka "x". Thus with volume pedal, in general, we are getting a "x - __" and all the way to 0 volume instead.
If theres any cases of changing volume pedal and the volume seem to increase, perhaps the increase is not much of a increase but with old volume pedal, its impeding the signal amount of whats already there in the first place. In a way, the increase is not increment of the signal itself, but the path which signal flows, is already widen(, thus allowing whats all present before, to come through.)