Hahaha ! 
A "downcountry" bike downsized with a 120mm stem ? I think it is the worst idea for both handling and power.
As acedeuce802 said : The slack HTA, steep STA, long reach, and long wheelbase all play together.
If you play around with that, it will be shitty for sure. Long stem with slack HTA is a terrible idea for downhill handling.
As well as saddle set up backward is a bad idea for uphill.
Back 10 years ago, every body were riding 650mm handlebar. On modern geo 29" bikes, nobody would do that.
You are entering the future of xc geo, do not be afraid, forgot your old bike geometry and take a L you won't regret it 
PS : spanish people are not tall. A L in Spain is a M in France and S in Denmark 
Is not my idea to mount a 120mm stem, according to my measurements 70mm would do.
Did you ever tried a longer stem on a bike with slack angle? because IMO there is a lot of bro science about geos, most important thing on a bike which needs to climb efficient (anybike at some point) is the saddle position, if you shift your saddle forward you are going to have less weight on the rear wheel and also less traction. I don't get the point to change my actual saddle position which I find perfect for power/comfort/traction efficiency. I also never had an issue of my front wheel wanting to rise on steep climb so I don't need to put more weight on the front when climbing.
Then it comes descending which you never do on the saddle and you want a longer reach from the BB to mount a shorter stem and a slacker HT angle and is what this bikes offers.
This frame is way longer than any other bike (even the new modern geometry bikes) and the only measurement which can relate it to a M frame is the seat tube lenght, which is also the less important as you can play with the saddle height/offset and that's because the bike has a really steep slopping.
By the way, is pretty stupid to say that because the people in Spain is shorter the size is different when all the rest of the brands keep the same sizing chart.
Do you race XC or XCM? the demands of those races are different to the recreational normal riding, I have to be able to put a high amount of power for a long sustained time to go up a climb. Already tested with different saddle positions and if I go to much over the BB, on long races my knees start hurting. It's the same comparison between a road and a TT bike, on a tt bike, because you lean over the handlebar, your position is way more in front of the BB, which usually drops the power you can put on the pedals but allow you to be more efficient in terms of aerodynamics which makes you go faster on the flats, but as soon as you start climbing and the speed is lower, you want to have the most efficient and powerful psotion (otherwise they would ride TT bikes all the time at the Tour de France).
I also understand, if for your kind of riding you want to sacrifice some climbing efficiency to get more of the bike going down, is fine for me but is a different way of riding the same bike.
If you check the Unno Horn's geo (created by Cesar Rojo, pioneer of all the new modern geo bikes) has a shorter reach, shorter wheelbase but the same 440mm seat tube lenght and 67 degrees HT.
Same with BMC fourstroke, Mondraker Podium, etc all of them modern geo bikes and this frame in M has a longer reach and wheelbase than the others in L.
