The humans become Titans through the use of an injected medicine, and the Titan form they take is due to the type of medicine injected. Therefore, it can be deduced Bertolt Hoover got an injection that gave him the gigantic form.
Case evidence is Rod Reiss, who became a Titan that is even bigger than Bertolt after he licked the spilled injection that he was trying to give to Historia.
If so, does that mean Historia Reiss would have become a titan of the same size (or maybe even bigger since she would have been injected with the whole serum unlike Rod Reiss that only licked a bit) as her father?
That would kill Eren and Rod seeing as everyone died when Ross transformed except the ones protected by the hardening ability and Kenny who managed to get out but died short after from injuries as seen in the recent episode.
She wouldn't even be able to eat Eren that way and take his titan shifting power and the coordinate since he would be vaporized by all the heat and burried under the wreckage. So she would stay as a mindless titan.
I don't think Rod would want to die by it too so either he didn't knew it would be that big since the other times the ceremony happened the titan was smaller, or she simply wouldn't become that big (for some unexplainable reason or something that I'm not aware of).
In the same topic, from the manga spoilers that I have seen:
If the mindless titans are in fact Marleyans traitors injected with the titan serum, shouldn't there exist some (during all these years) that would also become extremely bigger than the usual ones and become a threat to the walls?