The size of the blob which gives the maximum response varies depending on the algorithm. Also some methods like MSER[1] might return the area of the blob itself. I think an explanation of why the radius of the blob varies with `sigma` would be too elaborate, which in this case would require discussing the discrete first order derivative.

[1] http://cmp.felk.cvut.cz/~matas/papers/matas-bmvc02.pdf