In Attack on Titan, Rod Reiss becomes a disgusting abnormal.
By some research, I found out that the reason for his large form was that he took the serum in the wrong way, i.e., he licked it. Later in the episode, we see that Eren also takes the Armor Serum by breaking the vial by his teeth and taking the serum via his mouth - which is again the wrong way.
Then why didn't Eren become abnormal like Rod Reiss?
First of all, Eren was a titan shifter, so he cant go abnormal after being shifter (i believe).
Second of all, we dont know exactly if the power injection Eren uses are turning people into titans.
And third, Rod Reiss mentioned he cannot enherit Founding titan, that's why he was forcing Historia into doing it - we can assume he knew he will become abnormal after injection. So there's something special about reiss that makes him abnormal, diferent from other people like Eren or Historia.
But how did he know that? Why he became abnormal? We can't tell right now. Well, there's a lot of mystery around Abnormal titans and it's not yet explained. We only know some titans are more intelligent or behave in different ways or are looking very different.
There's wiki article stating that titan shifters are abnormal too. So its possible that abnormals cant become titan shifters (cant become "double abnormals"). So Rod knew somehow he will turn abnormal and won't be able to enherit titan power.
My guess is that the Serum only allows improvement of a titan's abilities, just as how Grisha was able to have 2 Titan powers without turning to an abnormal titan and still keep his original titan form.
It's unlikely the reason his titan was so big was because of the wrongful injection of the serum. The serum is called "Strongest Titan" so it's likely that's the reason why its so big.
