In my first experiences with two HomePods paired in stereo, I also had the impression that the sound level was less than the two when both were being used together (but not in stereo). I have them flanking my iMac computer so this is my normal setup when listening to music while using the computer.
My interpretation is that this is to be expected, as others have also suggested. When using the two speakers as mono speakers, you get something like 2 x the sound level of a single speaker. They are both putting out exactly the same sound and at the same sound levels (if that is how you have their volumes adjusted), and so you get louder sound than a single speaker.
When they are used as a stereo pair, it is quite different, I believe. The sound is now being divided between the two speakers. It makes sense that they have a lower total volume level possible.
I don't consider this a disadvantage. They play loud enough for me, and the greatly improved sound experience with the speakers as a stereo pair makes the experience considerably improved from the non-stereo HomePods, whether using them singly or two speakers together (in mono).
I think that you could easily do some experiments with this using iTunes, since it gives you the ability to operate either way. I may try it myself later, so if my impressions are different then I'll give an update at that time.