Note: The budgets are there for a reason, to keep players from building overly large settlements that could negatively impact performance.
See for commands you can use to see/get limits on triangle calls and draw calls. Spectacle Island lets you build up the most).
Some areas have small budgets (Jamaica Plain comes to mind), some areas have larger (e.g. I suspect that the build limits also relate to how busy neighboring cells are, since it needs to load all the polygons and actors that could be relevant in the area. All those conduits, lighting fixtures, decorations, crates, etc all apply to the build limit so you start with a fair bit of it used.
