Quite a few different potential variables here - a mass produced amp might be designed around lower voltages with longevity in mind; especially taking into consideration the smaller/cheaper components likely used within. Also, the speaker used can make a big difference in volume (or at least perceived volume) too... some speakers may have heavy doping or designed to be less efficient (lower sensitivity; either intentionally to get a desired effect, or potentially due to cheaper construction).
For beefier transformers; bigger transformer shouldn't directly correlate to volume, but rather more so tone potential (and some level of interactivity with the power tubes and distortion). A smaller or poorly designed transformer can crop off or fail to reproduce as wide of a range of frequencies... of course, we could be teetering into cork sniffing territory here... in some scenarios, size doesn't really matter and/or a smaller OT might be intentionally used to get a certain desired effect (so bigger doesn't always necessarily mean better). The construction/design of the transformer may have a more noticeable impact - a mass produced amp could use an output transformer with a winding ratio designed to not push the power tubes too hard (for longevity and/or less distortion), which could mean potentially lower volume (like using a 5k primary/8ohm secondary instead of a 3.4k primary/8ohm secondary). Again, lot's of variables here, just sharing some overly generalized feedback.
Again though, lot's of potential variables though... sometimes it might just be some boutique goodness coaxing you into turning the volume up just a little higher.