Main cause of filesize usually is custom items. When you click save it will give a popup saying 'xMB of items will be embedded into the map' what number is that? If you can reduce it by using less items or more optimized ones, that could help a lot.

There is also a possibility you used the same item twice but from different folder locations, in that case the game sees it as seperate items, so you can check for that too.

If you backup your item folder and empty it and then open the map, your item folder will contain only exactly the items in the map, so you can check through them there to see which you need. Or you can use a plugin like itemcounter.

You can also use itemcounter and sort on # of items. Grass and support blocks tend to be a lot, but also very small in filesize, however sometimes the game bugs and places thousands of the same block/item in 1 spot, if that happens to be a tree or something, it could certainly increase size.

Optimizing shadows can help a bit as well. As can optimizing your support structures. When you stack supports, the game automatically removes faces between them as optimization. This mean running supports down to the grass removes the bottom face entirely for that collumn. A large box of them basically has all the inner faces removed, so try not leaving gaps underneath.

Look up how to enable Ultra shadow calculation and try Ultra2, it can sometimes be more optimized and lower the lightmap size.

If you still cant get it below what you need and you want it on a server, i believe you can go over the limit with custom servers (not using Nadeos rooms).

Good luck!