It's kinda melancholic to see how the two have grown ever since they met at the rabbit house...
Agreed - and it's true of all the original girls, really - I noticed it the most with Rize, but also Cocoa, Chino, Chiya, Syaro, Maya, and Meg have grown up a lot over the course of the story. Even the Bright Bunny girls and Yura have matured in their relatively short time in the series, though it's not quite as easy to spot.
Most of it's in the characterization (since the characters were first introduced, they've overcome a lot of grief, loneliness, anxiety, and/or insecurity, for example), but I don't think the visual part of the impression can all be blamed on the art shift over time, nor simply on different hairstyles and outfits, but I can't really put a finger on exactly what the artists are doing differently to make the girls look older now than they did at the start of the manga. Whatever it is, it's subtle enough that I barely noticed until I reread from the beginning, but effective enough that it's convincing.
There's definitely a melancholic effect to it - not at all a bad thing, but I almost miss them already. The story has been moving along a little faster than I realized - looks like Syaro and Chino are already studying up for their college entrance exams, with their graduation (and Cocoa's) only a short time away!