Because the Bionic still uses the first generation of LTE chips which already drains too much power, so not like comparing to the D3 or even the DX2.
Second generation LTE is supposed to be on chip (stressing "supposed"), so given the dev lead-time, it is obvious the Bionic does not have a second generation LTE chipset since using the 4430 or 4460 (they do not have LTE on chip).
Point is with LTE, battery life is short enough, so any percent less is too much. Unless you leave it off. Still your point is correct, since it will not cut the power in half.
Added:
BTW, the size difference is not proportional, since more than one dimension. For a 16X9 shape, the diagonal needs to be multiplied by .49 for width and .87 for length. The surface coverage is the relative point for battery consumption, so 4.5" vs 4.3" diagonal is actually about 9.4% more battery need in regards to surface space of display (same pixel total, but more light needed).