I presume the Physics is calculated on the size of each object when used. It has very little relevance in some situations to the actual weight an object might be as any size object could have a variable density or weight, but I guess a default calculation has to start somewhere.
Personally I would prefer it if no Physics attributes were set at on objects by default as there are likely to be as many that dont need it as do if not more, though at least it gives some reference point to start from as otherwise one would have no idea what numbers to start with unless of course one reads the manual which many dont seem to do.
It is recommended I believe by TGC that the default settings are reasonably accurate and altering them can have unpredictable results and this can be true in some cases. In others changing the settings are necessary - for instance I set characters Physics weight and friction to 500 as the higher default figures stop them moving correctly as the default settings seem to be a drag on them and they cant move around well if at all depending upon their location.
Just my findings
