Subject Combination: seeking for advice.

Hi. i want to know why the percentage for taking F4 & F9 is so low? Actually, at first i’m taking F4, F5 & F6 this coming december but i eventually drop my F4 because is too heavy for me as well i hope to get higher marks for every paper that’s why i made this decision. So, in June 2015 i will be taking F4 & F9 is this reasonable? I wan to seek for advice. thank you.