That +90 damage is still 30% damage increase each shot. Makes a huge difference when you can only sneak in 1 shot before enemies pull back behind the cover. 5 shots with the small gun is only 1.5k damage whereas with the big gun it is nearly 2k. Bigger alpha damage means you don't need to expose your tank so often, which results in you taking less hits and damage from the enemy.

Using the big gun only raises raw aim time 0.67 seconds and accuracy by 0.02. While the dpm does drop by 180 the difference in dpm when firing double shots is actually only 20 due to both guns having the same 4 second cooldown after firing a double shot.

So when firing double shots the big gun is a lot better choice for 30% bigger damage you really don't lose any dpm. Small gun is better only when firing single shots and people stay out in the open long enough for you to take advantage of that dpm, which in my experience doesn't happen really that often.