On-disk temp tables: suggest too small values?

[tobiasKaminsky]

On my private instance and tech-preview, I get:

On my private one I am already using:
tmp_table_size = 128M
max_heap_table_size = 128M

On TP:
tmp_table_size = 64M
max_heap_table_size = 64M

So 64MB seems to be a hardcoded(?) value, but should probably be more?