It looks like the size increase is due to debug informations in the ELF file /usr/lib/gcc/i686-linux-gnu/6/lto1. (Same for cc1 in package cpp-6.)
The debug informations do not seem to be mandatory for a proper use of gcc. Could this be split into a gcc-6-dbg package in order to reduce gcc-6 to a saner size? Best regards, Celelibi