Apparently, NES-ETROM and HVC-ETROM boards can support both 8kb / 64 kbit (6264) and 32kb / 256 kbit (62256) chips, selected by solder pads. This allow this board to support 16 kb total RAM (two small chips) (the only possibility actually used by comercially released games), 40 kb total RAM (one chip each), and 64 kb total RAM (both big chips, suspected to be used by the Sim City prototype).
However, it looks like NES-EKROM / HVC-EKROM boards only supports small SRAM chips (8 kb / 64 kbit) while NES-EWROM only supports big SRAM chips (32 kb / 256 kbit). If it is simple to make a board that support both chip sizes, why are EKROM and EWROM different boards in the first place ? They could have made just a single board that supports both sizes through solder pads.
The only real difference seems to be with pin 26, which is Chip Enable on 8k SRAMs and A13 on 32k SRAMs.
However, it looks like NES-EKROM / HVC-EKROM boards only supports small SRAM chips (8 kb / 64 kbit) while NES-EWROM only supports big SRAM chips (32 kb / 256 kbit). If it is simple to make a board that support both chip sizes, why are EKROM and EWROM different boards in the first place ? They could have made just a single board that supports both sizes through solder pads.
The only real difference seems to be with pin 26, which is Chip Enable on 8k SRAMs and A13 on 32k SRAMs.