Try filling your mouth with water and then trying to push it out through a straw. Now try cutting 4/5 of the straw off, and repeat the experiment with the much shorter straw. Barely any pressure is needed for the short straw while you'll find it requires a bit more pressure with the longer straw. In a sense, males need to push a bit more to overcome the pressure, while females may end up with the same results from just allowing their muscles to relax.

For the non-ELI5 answer: Female urethra is generally a fifth of the length of the male urethra. In addition, the rhabdosphincter (external sphincter muscle for urethra) is significantly thinner in females. Because of the above stated, females would require less pressure to void the bladder.

Here is a relatively new (within the past 5 years) study talking about the differences between male and female lower urinary tract biology and physiology.

As well as the Wikipedia pages for both the male and female versions of the external sphincter muscles.