I thing that would fluctuate drastically between differmt toa or matoran.
For example, the Toa Nuva would weigh much more than their previous Toa Mata forms, simply due to the new big pieces of armor.
The Toa Inika are apparently freakishly tall compared to other toa, so they would probably weigh a bit more to.
2001 style diminished Matoran would weigh significantly less than say a Metru-toran would, simply because they're so much smaller.
The only thing I could imagine being consistent are Turaga, seeing as there's only ever been eight official Turaga sets in all of G1, and they're all of roughly the same size.