Feel free to correct me if I just did not figure it out, but to me it seems like there is atm no way to increase the Utility Cabinet inventory size.
I do not mean radius aka. covering more ground for the base - I am talking about how many materials one can put into the UC. When playing with friends and having a few more (as in, decorational too) items, the UC gobbles up a whole lot of material.
If you can not log in every day to refill the UC, your base starts to decay. I would love it if you could increase the inventory size to have more time in between refills - like, if you are on holiday etc.
(I tried several different ways with more than one UC next to each other, filling them with the same materials, filling one with wood, one with iron scrap etc. - does not seem to work for me. The game does not seem to "know" that there are several UCs in the range from which to take material.)