These are very different weapons systems. The Autocannon is a short range, direct fire weapon designed to be mounted on a manouverable Battlemech, whereas the Long Tom is a long range, indirect fire artillery weapon mounted on a large, slow, tracked vehicle.
The Autocannon/20 rating actually covers a range of different weapons with varying caliber and rate of fire, however the "20" rating means their damage output is equivalent. In the sarna.net description for example the "Crusher Super Heavy Cannon" is a 150mm caliber weapon that fires ten shells per "round" of ammo (i.e. each individual shell is actually 20kg, not 200kg).
The Long Tom on the other hand fires single very large shells (200kg per shell) that are large enough to have an area of effect when detonated (hence the varying damage depending on distance from the centre of the blast). The potential damage from being at the centre of the detonation of one of these very large shells is therefore much greater (30) than being struck by a number of smaller Autocannon shells (20).