Ever since the truth about how titans are made was revealed, I've always found it interesting to compare their human forms with their pure titan forms, especially since they can reflect the Eldian's physical and emotional state at the time of their transformation and even a little bit about their personalities
Take Eren for example. His pure titan is quite small, about four meters, which is fitting since he was a child when Grisha injected him. But his mouth is also torn open into a gaping, lipless maw. Not only was he screaming at the time of his transformation, but he's also an angry kid in general. So it's fitting his pure titan would be a little rage gremlin.
I would say that probably all of the Warriors were around Eren's size when they became pure titans since they were all around the 10-12 range, if not for Falco who was significantly taller. Although that could just be because Zeke's spinal fluid did a bunch of weird stuff to Falco. Falco is an interesting case where his pure titan doesn't so much reflect his personality, but a theme. That being the life cycle of a bird.
Falco's pure titan is modeled after newborn birds, which are rather notorious uggos in the animal kingdom. Then his first Jaw Titan is a fledgeling and then... well, you get the idea :)
I've already done a theory about how the more animalistic pure titans reflect how they might have been in human form, but what I didn't get to mention is how that applies to Ymir.
She has the same build as her Jaw Titan form, long arms which she walks on like an ape. A big part of Ymir's backstory was how she was groomed, lauded, and presented to those cultists as the second coming of Ymir. Being gawked over like a monkey in a cage, or better yet a trained monkey (very much like Zeke being a trained monkey for the Marleyan military). I know it's kind of a tenuous connection, and I will say that I also like the popular theory that Ymir Fritz designed our Ymir's titan to resemble the storybook version of the devil she "made a deal with" as sort of an ironic homage.
Ymir's titan having a permanent scowl may reflect the deep anger and resentment she felt at having been sold out by the cultuists, at Marley for punishing her for something she had no say in, and at herself for taking the fall instead of saving herself when she had the chance.
And finally, there's everyone's favorite deadbeat, trypophibia-inducing titanic grub, Rod Reiss. I opted for one of the less disgusting puctures here btw, you're welcome.
We have the clearest answer for why his titan is the way it is, he literally slurped the serum straight off the ground and got smushed down by the cavern walls while the titan was still forming. But on a symbolic level, it fits how Rod really is this pathetic worm of a man who has nothing left to do but come groveling back to the one family he has left, not to make amends, but to make a grab at power, hence the long arms. His intestines falling out at Orvud district? Gross, but also shows how he's both literally and figuratively gutless.
The titan's bulbous body reflects how Rod was pretty out of shape given his age and indulgent lifestyle. And his titan repeatedly getting its face shredded off from friction matches up with how he's essentially lost his identity to the royal family's doctrine. Not because of Karl Fritz's will like with Uri, but he had given up trying to fight it just the same. Also I might be reading too much into it, but he do be looking kinda phallic and Historia's entire existence is attributed to this man not keeping his dick in his pants.
