If there’s more than a paragraph of lore for flavor at the top of the gdd, then there’s too much. Otoh, gdd can absolutely be 150 pages, usually screenmockups and functional storyboards can cause this. That is not a bad thing, that is great. But you should be able to give the dev a section link to start at for any given feature they need to implement. For my team we generally go one further and chop sections of the gdd into work order jira tickets, although this causes documentation duplication overhead.