Cross-country MTB riders always go with small frames if they are serious about it, because it allows them to drop back behind the seat on steep descents and makes the bike easier to handle in tight technical stuff. I have been riding a Marin I bought new 25 years ago and I just rode a very technical single-track trail with it that hosted a race a few days before, and it felt perfect as usual. I could ride an XL size frame easy, but when I bought this luckily I researched it and bought the L size frame, I am about 6'2.5". This is what my bike looks like currently;