User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5020152-20130305143423/@comment-5043481-20130306161407

For the aquati monsters, they must have densities similar to that of water, or else they would sink, or float. Taking lagiacrus as an example, he is 30m long, and about 2m in average diameter. His volume then works out as 190m cubed. water is 1000kg/m cubed, so lagiacrus has a mass of about 190 metric tonnes. Thats quite big.

There's no way a rathalos has a mass of 400kg. More like 4000.