More info is really required to actually provide any guidance as there are so many prongs to how you could end up in this situation.
Due to the nature of C# you're always going to end up with oodles of DLLs for all of the libraries that you're using that aren't system. With something like Fody.Costura you can compact them all into your executable w/ compression (you have to turn it off on C++/CLI libraries though) at the price of making NGen laboriously slow if not done during install (affects build→debug cycle terribly, I've experienced 5-minute stalls due to Costura being naively enabled on debug builds for a while).
Might want to post a list of all the DLLs in case you've done something funny and you're pulling runtime assemblies into your stuff (instead of bundling the runtime into your installer like you're supposed to).
The other common cause of DLL bloat is the ecosystem nonsense NuGet has created. So you'll have SuperSocket but your use of SuperSocket is actually 6 different assemblies for specific functionality, I go through and pull those kinds of projects myself from git and amalgamate them to keep that garbage tapped down or I ILMerge them, in which case I NuGet pull them into an isolated project then ILMerge them and bring them into my project. Seriously though, NuGet is how you get into this situation in the first place and nuget really needs to add some capability to do these sort of compoundings on its' damn own.
The other solution is rely on fewer libraries and write narrow solutions that fit your exact problem instead of pulling in a gigantic Tween library of which you use 10% of. If you look at a lot of your library inclusions, you probably don't use a lot of what's there and can probably find a more compact JSON library, that doesn't have a lot of bloat for that silly JSON query language that pretends to be like XQuery but w/ the caveat that it is absolute garbage, that you don't even use.
If you're building installers there will also be compression there that will make a difference, some assemblies will compress by as much 1:4 IME. So transmission won't be as harsh as storage, usually.