VisualGDB takes the memory sizes from the linker script file included with mbed (<BSP>\mbed\targets\TARGET_NORDIC\TARGET_NRF5\TARGET_MCU_NRF52832\device\TOOLCHAIN_GCC_ARM\NRF52832.ld). They may not match the actual memory sizes if the script reserves some of the memory for other components.
Based on our knowledge, mbed support for nRF5x devices requires some extra build steps to combine the image with a bootloader and VisuaGDB does not do it automatically. As a workaround, please try creating a regular non-mbed project until we fully support targeting nRF5x with mbed.