If the NA (in vacuum) tends to the limiting case of 1.0 the maximum value of |E|2 increases faster than expected from the scalar theory (Airy disc) with a maximum deviation of about 13%. The second effect really comes from very short pulses, i.e. very small values T. Then, the value of |E|2 compared to the expected linear increase with 1/T decreases slightly (only less than 2%), but systematically for all NAs.
Even more interesting is the dependence of the height of the first secondary maxima along the x-axis and y-axis on the NA and 1/T. It can be seen that along both axes the first secondary maxima nearly vanish for very short pulses, i.e. large values 1/T.