Is the build size equal to the sum of the size of the assets?

In my game I am using a asset which is 130 mb big and I am adding more things so I want to know that when I will bulid the game / publish the game so the size will also that much big?

That depends solely on what assets you use.

→ If your asset is a design-time tool, such as a custom editor or program that will run inside unity but is only used to improve workflow or supply any other editor-time functionality then these assets themself will not be included in the build.

→ If your assets are design-time tools as above, but also offer functionality used inside of the game (think Odin Inspector that comes with Odin Serializer) then that asset will probably add some custom libaries to the build, the main tool will still not be in the build

→ if you add an asset which is a fully decked out terrain, including audio, high quality models and 8k textures totalling 5 gigabytes then that will be added to your build completely.

So … it depends

No, the build size will be significantly smaller, like you could use Gaia which is over 1GB and model.your terrain and the app could be under 50 MB so don’t worry too much with the project size