It doesn't fly MEL-BOM today. It flies from about 230Nm further east. Hence the payload limit.
230Nm is good to carry roughly an extra 40 pax and, given the configuration at the time, that would have allowed virtually a full load.
Try it at
Great Circle Plotter
Note, it's also interesting in the context of this thread to compare BKK-LHR to SYD-BOM. Shows BKK-LHR may not have been quite as limiting as suggested earlier.