Usually the landscape is the biggest contributor depending on its size and lightmap resolution, not sure why the landscape isn’t visible in that matrix. 280 MBs is completely fine for a map in my opinion.
Textures, especially lightmaps will increase the file size a lot. You can reduce your lightmap sizes on some areas by making modular planes (disabling on some of them will work too) or you can use dynamic lighting to get rid of those lightmaps if it fits to your project. For me 280MBs is fine too because you are using lightmap for whole map, instead of gameplay area.
As I’ve mentioned, the biggest contributor to lightmap according to the statistics is 38MB… not even close to 280MB.
Total of entire map’s Lightmap should only be 50MB.
Based on this I believe it isn’t caused by the lightmaps.
Hence, I’m a bit curious as to where the size comes from. Or perhaps the Statistics does not fully describe the actual size?
Is this cooked map size of editor map size? The uncooked version of the map will include all manner of additional editor-only data that will be removed during cooking, and likely isn’t included in that matrix.