The behaviour of $F(n)$ varies dramatically with the prime-factorization of $n$. Typically one gets much larger values for $F(n)$ when $n$ is the power of a prime, particularly when that prime is equal to $2$. The key conjecture in this area (I believe) is due to [Higman and Sims][1]:

> **Conjecture** Let $p$ be a (fixed) prime number. Define $f(n,p)$ as the number of groups of order $p^n$. Then:
$$f(n,p) = p^{(2/27 + o(1))n^3}.$$

(The link above gives a more detailed version of this conjecture.)

  [1]: http://groupprops.subwiki.org/wiki/Higman-Sims_asymptotic_formula_on_number_of_groups_of_prime_power_order