Remember that this is an ‘except’ question. Four choices will give things that the author does suggest as an explanation for the numbers, and one (the right answer) will not. That the food contains a concentrated source of nutrients is given by the phrase ‘much nutriment in small bulk’ (line 67). Rapid regrowth is covered by ‘has a rapid growth' (line 69). Large area by ‘roam over wide tracts’ (line 65). Overestimate of requirements is covered by ‘much exaggerated’ (line 74). This leaves D as the right answer.