Looking at a w3wp Process (hosting multiple sites) in Process Explorer shows that it hosts several different application domains with different instances of the same assemblies loaded into memory. So moving the sites into a single AppPool may not help much.
But there is another option. In IIS 8+ you can share common assemblies across AppPools. If certain assemblies are used by multiple AppPools, they are loaded into memory just once and then aliased by the different processes.
Have a look at this bit from asp.net and this TechNet blog post
You have to do a little bit of setup work, but then it seems to work quite well.