The very smallest most portable subs can be found at Subway. It's a Friday

...
Ok, I'll get serious. The problem with going smaller is that one sub will not give you the low end or the impact of the beast you currently have. Therefore, plan on having a couple of smaller ones. If you stack them in a corner placement, you can get 6dB of room gain and it will help somewhat. What this does to the smoothness of low end FR depends on the room modes and their intensity.
The PB-13 Ultra is 27Dx20.5Wx22.5H" and
155lb. Here are a couple of "smaller" options that I have read positive things about,
HSU
VTF-3 MK4 25Dx17.0Wx21.5H" and
90lb
SVS
PB-12 NSD 22Dx17Wx21H" and
66lb