Higher volume makes the boat float higher while on its side which tends to make a "hump" you need to get over to roll up. Flattening the deck does make the boat more stable when upsidedown, but the hardest part of rolling is when the boat is at about 90 degrees. With the lower deck, the boat can sink down farther making it easier to get it under you.

Lower volume boats tend to be easier to roll, however it comes at the cost of lower secondary stability. It is the reduced secondary stability that makes the boat easier to roll through the transition.

: Increasing the volume by widening the boat will certainly make it harder to
: roll, but will making the cocpit higher have the same effect? It would
: seem the tall, rounded cockpit area would help the first half of the roll
: (upside down to halfway up)when it is submerged. It should be irrelavent
: for the rest of the roll, when only the hull is in contact with the water.

: Also, lowering the volume by flattening the decks would create a form that
: would be more stable (and harder to start rolling) when upside-down,
: wouldn't it?

: Anyway, for a given boat length and volume, the minimum height boat must be
: the maximum beam boat, and thus harder to roll.