I often find myself posting questions about problems and attaching a question like this one. What I keep trying to remind myself is that if I suspect that element X is at fault I should try a quick experiment which would tell me if I am correct. In your case I suspect that changing nothing in the scenario except for increasing POST_MAX would have given you your answer much faster than posting. I don't intend this as a flame at all but merely an meditation on saving myself effort (the virtue called laziness) which I myself need to contemplate.
hth
jg
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If it gets a little bit out of hand sometimes, don't let it fool you into thinkin' you don't care.TvZ