For a given MCU, all other things being equal, does the memory capacity impact the power consumption ? A 512kB flash memory would consume more than a 128kB ?
No. Flash requires no power to maintain its data. It only uses power when actively reading or writing.
The amount of flash has mo direct bearing on the power consumption of the chip.
SRAM, however, does require power to maintain the contents, so the amount of SRAM on a chip vastly affects the power consumption of the chip.
Take a look at the datasheets for the micros you are interested in. For example the Atmel XMEGA datasheets show that there are no differences due to the amount of flash memory, only the amount of SRAM.