So after noticing how similar Alien Bris and Waroga were I wondered, if they were the same species or if it is a case similar to Red where two things just look similar then I began to wonder... why are there no giant aliens in the manga. So this is my theory...
Now for this I have thought about why is it that giant monsters exist in the first place in the Ultra series and for that we have to go back to the Ultras. It was said on the Jap WIki that when the Plasma Spark was first activated it 'disturbed the evolution of nearby planets creating monsters' in the Story 0 manga the Plasma Sun (the counterpart of the Spark) also caused the first appearance of monsters all over the universe. What does this mean...?
Yes, Ultras are the reason for monsters existing, although I doubt the blame can be so unanimously put on them given the Plasma Sun only affect nearby systems, it shows that more often than not, Giant Monsters are not something that do not naturally appears which means that they are caused by outside intereference.
Now here is a question, how many of the aliens who were good could transform into giants? Because from what I know, usually you only see invaders turn giant, think about that. Now here is my real theory, what if turning Giant is not a natural ability for most aliens seen in the series, what if its a technology, artificially given power reserved for the military and/or high ups. It would make sense, if civilians didn't have access to something like that, that is was military hardware that was not easy to acquire. But then why do so many invaders have something like that, well, in the Ultra series, if you're an invader you are probably going to have to fight Ultras who are naturally giants or against invaders who use giant monsters. Given Earth's history with giant monsters and aliens it would make sense that the SSSP made keeping this substance/technology of Earth a very high priority.
So in an nutshell my theory is that the ability for aliens to change size may not be natural and only certain individuals within alien civilization can actually size change