The Abomitaball Snowman

Made from a series of metaballs plus a particle system emitting metaballs from a hidden mesh

1. Shape the static part of your snowman with a few metaballs
2. Add a mesh emitter object roughly shaped like your snowman body (you can probably make a copy of your metaball base and convert it to mesh)
3. Add a Simple particle system with Newtonian Physics, pointing to an additional smaller sized Metaball object.
4. Under Velocity turn off Normal and increase Object Z slightly. In the Field Weights disable gravity so they "float up", then add a texture to it.

5. Make the texture is Blend type, and turn off all influences except Size.
6. Switch Mapping to Strand/Particle so it maps out "through time" rather than any spatial coordinate.
7. Under the Color panel enable the Color Ramp and flip the default result direction so they shrink with age rather than grow.