There's a trend of adult female characters who are short and busty, Uzaki started it and Shiori is another example of it but while Uzaki-Chan is a massive tease and full of gremlin energy, Shiori is kind, responsible and absolutely adorable.

At a guess, Shiori is in her early to mid twenties, there are people who are not comfortable seeing the Uzaki body type, so when Shiori is introduced, she throws a curveball at you, you think she's just someone who is just cute and easily submissive but quite often throughout the anime she proves her maturity credentials, just by being in the office environment, you've already eliminated some of the ecchi elements that weren't always wanted in Uzaki-Chan.
If it wasn't for the fact that Tiny Senpai is a romance anime, Shiori would function perfectly fine as a normal office lady with an appealing design, she even out right states that she's never had a relationship and has no one outside the office trying to protect her or push her into it making her very independent, a rare trait among female leads, in fact the office itself never really crosses the line in the support Shiori and Shinozaki get for their blossoming relationship.