May be need wait patch for build process. It problem present only for first build after start Editor.(Editor create internal cache and make compiled texures). Read my topic about it - Big memory problem with Editor 2 (SOLVED)
May be it help you 
Really after build complete and free memory Editor aquired about 1-3Gb memory only for big collection and good work. Build process fast too and use huge memory amount.
About images - try only drop transparent pixels and not change image size.