It might reflect a relatively small amount of material - one theory for the eruption is an earthquake or initial explosion caused part of the island to slide in to the sea, which allowed sea water to mix with the revealed magma and that went 'bang'. In that scenario there'd be a lot of steam, some ash but maybe not much more than that? I guess it could also have been a similar thing with part of the volcano that's underwater.
There's a really cool radar image of before and after the eruption on the BBC:-
Pacific volcano: Ash-covered Tonga is like a moonscape say residents - BBC News
I know a large part of the island was relatively new and made of ash etc. from earlier eruptions so not exactly durable but even so it looks to have essentially destroyed it.
That said a VEI2 does still seem a little low given how large the ash cloud was and how high in to the atmosphere it went
Originally Posted by: Hippydave