Only a guess atm…
The linker script that bluepill_f103c8.json sets the flash memory size, and bluepill_f103c8_128k.json does not have a valid linker file at present. I do however suspect after a quick look at the stm32duino to so how they account for 64K and 128K that a valid version of the linker file just needs to be changed to specify 128K, instead of 64K as it currently does. i.e, a copy of this file, and the relevant changes made to the 128k json file to point to a new linker file, and the relevant line changed there.
However, when I tried exactly that, I got a Warning! Cannot find linker script for the current target! and the compiler now has stage fright, so I got no idea what I missed editing there… I thought it was a matter of ‘create missing linker file with minor change, update path to it in board json, job done’ … but no, it wants to be … DIFFICULT!
Not to be dissuaded, I decided to go about it entirely the wrong way, and edited stm32f103x8.ld so that it said FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 128K on line 4, and line 55 of bluepill_f103c8.json to read "maximum_size": 131072, … and well, yes, it did compile and say the right things… but that was entirely the wrong way to do it! 
btw, there is no reason you couldn’t do the same to genericSTM32F103C8.json … I just happened to be poking around in the bluepill_f103c8.json. But the real question would be… does it blend… er, sorry… does it still work!?
PS: these files are located in %userprofile%\.platformio\platforms\ststm32\
