People are dumb. If you take out the army builder and the army lists and the fiction, the rules are small. If you leave out vehicles for your first couple of games, it's 40 pages of actual rules (I skipped the large contents section). If it went down the path of only explaining the common rules, then tacking on all alternative movement and fire options as "special rules" for the appropriate troops, it'd be even more compact. That kind of approach is what makes "Song of Blades and Heroes" work- it masquerades as simple, but really, it isn't.
Defiance tends to play out very fast for games with 20-30 figures and 1-3 vehicles per side, which is pretty big for 28mm skirmish. It's unpredictable though, games can drag out in very heavy or very open terrain, where you can't maneuver or where everyone's too chicken to stick their head out into the long, unobstructed fire-lanes.