Backpack size - 10L vs 14L
I've got a Cambelbak MULE which has about 11l of pack capacity. I found that this is quite enough for day-long rides, far from any roads. I packed:


tools
spare tube
patch kit
pump
a spare tyre sometimes
some food (energy bars and similar low-volume-high-yield food)
small first aid kit
rain jacket
a warm undershirt if necessary


My fullface helmet could be strapped to the outside on long ascents. If you're feeling adventurous, you'll even get full size shin/knee guards strapped to the side of the pack, but then it will be a little unwieldy.

If you don't need to attach your helmet to the pack, you can roll up clothes and attach them around the sides and upper edge of the pack using the four compression straps.

Beware of one thing: If you fill the bladder up to the maximum, the back of the pack will bulge out, reducing comfort.