65 tonnes = 65 cubic meters. Or a swimming pool about 5.5m x 6m x 1.96m deep (water is pretty massive). Doesn't seem like a lot to me, given the mass of the main gear trucks (~6.8 tonnes each).
Plus it occured in local tropical summertime. Daily high temperatures of 28°C - the water may have been 25-35°C after sitting in the sun all day, and thus less efficient in absorbing and removing heat from the metal/rubber/carbon of the gear.
One would have to juggle starting temps, specific heats, joules, volumes/masses and such to get an estimate of water required to achieve cooling of xx°C. And then factor in inefficiency of delivery (water that just sprays on the runway).