How is it possible to include so much information in such a short essay?

Precision and simplicity are what GCSE exam boards are looking for. You only have to look at the mark schemes to realise that it is very much a checking systen - there are specific points which the examiner is looking for and where included will guarantee a mark, so long as they are backed up with relevant evidence. For this reason, it is often useful to look at the exam board's mark scheme and their sample questions to see how it is possible to get all the points down within a very short space of time.

While obviously facts are extremely important in a history essay, you are not reciting a story, but creating an argument. You have to be concise - no waffling, get straight to the point, give evidence and move quickly on to the next. PEA - Point, Evidence, Analysis.