I tried a quick 1 cycle each of a T1 small mining laser and a T1 medium mining laser and was completely surprised to see they each mine the same amount of ore per cycle.  I think the medium lasers should mine slightly more per cycle than the smalls.

The calculations below are based on the default stats for a standard small mining laser and standard medium mining laser.  These take no skill boosts into consideration as the relative % should remain the same since both lasers are effected by the same skills.

CPU-- Sm=40, Med=50...Med is 125%
Reactor-- Sm=30, Med=150...Med is 500%
Mass-- Sm=400, Med=1000...Med is 250%
Accu-- Sm=17, Med=85...Med is 500%
Cycle-- Sm=18, Med=12...Med is 33% faster than SM.

The fitting requirements for a medium over a small are IMO to much for only a 33% increase in cycle time. 

If you compare this to weapons a medium weapon does more base damage than a small(small standard laser 100%, medium standard 138%) and the ammo does about twice as much as the smaller version.  There are no medium mining charges which would increase base mining yield per cycle.  Giving a small increase to the laser itself would make sense, though I'd love to see medium mining charges added instead/as well with 2x base yield of a small in keeping with other ammo.

I haven't tested it but all the above possibly applies to harvesting as well.

I'm running the Lvl 1 transport missions and stockpiling the ammo.  I'm looking for a laser user who is doing the same and would be interested in a straight swap my Small Volcano Energy Cells for your Small Smite Ballistic Missiles.

PM me in game if you are interested.