How to get double loot and XP from a Pokestop?

Loot is just random. sometimes i get 3 items, other 7. XP in my personal experience is related to the number of items you get. 100xp with more than 5

source: hours and hours of playing

I have found that every 10th unique Pokéstop spun will give you 6+ items, 100 XP and an egg (if space permits), but you have to follow a specific pattern. Basically, if you continue to spin unique Pokéstops, every 10th stop will give you the bonus. Someone test this out for me.

If you get multiple xp gains in a short time, the game will only display the total once. It may be that.

Look in your history journal if you want to double check.