Just finished Mission 3 and the map size is insane!!!
I spend 4 hours last night just on that one map and I still only covered maybe 2/3's of it at best! It was also the first "long distance" map, where missions 1+2 had most (but not all) of the combat taking place <100m and mission 3 immediately starts with dozens of 450m shots if you want to play that way. The map is so big that it becomes a bit of a problem for the stability of the game as I had my first bunch of game-ending glitches occur - Binos stopped working (no zoom), unable to fire weapons whilst "aimed" but fire from 3rd person was OK - all of which still occurred immediately after a reboot. The binos one never fixed itself but the "unable to shoot when aimed" was semi-fixable by taking an shot in 3rd person first, then aimed fire would work for 3-10 shots before the glitch came back.
I don't how much of this is down to map "size" or how well it's been filled with enemies, as I think I easily killed 150-200 in that one mission alone and, for a sniper game, it just became a meat-grinder slug-fest really quickly once you got to within 100m of the objective (a massive cathedral) and my stats at the end which had been 100% Lethal, 50% stealth in mission 1+2 dropped into 100% Lethal, 75% Assault as Stealth stopped becoming a realistic option due to the huge number of enemies closely stuffed together.