system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some upper case letter just a bigger version of their lower case (c to C, k to K) and some are completely different (a to A, r to R)?
gpt
Different reasons. As you probably know, romans didn't have lower case letters at all. So they didn't develop simultaneously with the upper case ones.
Greek, on the other hand, does use lower case letters, so they got adapted into the alphabet that we use now (with varying levels of accuracy. Rho looks like p, but has nothing to do with it, for example). 
Some letters, like R for example, used to look the same in lower case, but "evolved" over time into something that was more efficient for writing quickly by hand.