Well the weight of the BDP300 is doubtless because it's pretty similar to the 100 - and so looks like this:
Where as a properly engineered Arcam FMJ like the 139 looks like this:
(Using Crustyloafers image from 2006).
Arcam appear to have taken quite a lot of 'stuff' out of their new players. Not quite like for like of course as the BDP's frustratingly to many of us are not universal players, but whether or not the electronics removed were needed to maintain performance quality is the debating point. General consensus after the dust settled was that the BDP100 was not the equal of the DV139 sonically, so therefore it looks like the gubbins they have not included in the BDP's is in fact doing something other than just increasing the weight!