Back in the 90s, when I was actually racing mountain bikes and riding aggressively off-road, I liked small mountain bikes. I am 5'10" tall, yet I rode 15" and 16" frames, mostly. I found that I could flick them around better, and I enjoyed the extra stand-over clearance that the shorter seat tubes allowed. Once I figured out to use riser bars on the 135mm sems that the short top tubes necessitated, I even stopped going over the bars at the drop of a hat. (I could have used a higher rise stem, but I just didn't like the feel of that, for some reason. Of course, even with the riser bars, I had the grips 3 to 4 inches lower than my seat...)
As I got older, and my riding style changed, I found that these smaller bikes seemed cramped. Even my 54cm road bikes began to feel a bit on the tiny side. So, I gradually moved up to the medium-sized frames, 17" or 18" for a mountain bike and 56cm or 57cm for a road bike.
When I designed the ti Funk "do-everything" bike, I based the measurements around a 1988 Specialized RockHopper in the 19" size, combined with a 57cm road bike. So, it has a 57cm seat tube and a 56cm (22in) top tube. There is no terrain over which I will ride a bike, nowadays, where these dimensions don't work for me. It has plenty of room to stretch out for long road rides, and enough standover clearance for any technical terrain I will currently attempt to ride.
Now, when people ask me what size bike would fit them, I can no longer give them the short, racing-oriented, answer that I was taught to give in a retail bike store. I have to quiz them about how and where they will ride, what their goals are on the bike, and how aggressively they want to compete with their companions.
Keep that in mind when you are looking at bikes. One person's "too small" may be another person's "racing" size. And another person's "way too big" might just be your "comfortable for long hauls".
There is no universal answer to bike sizing, and anyone who tells you that there is is only trying to sell you something (a bike or their outlook, perhaps).