Did the contractor actually _measure_ the load on the various phases, or was this calculated?
What is the THD of the ballasts on these fixtures?
Given your description of the neutral bar heating, I'd urge checking for harmonic neutral currents. From your description, these ballasts are fitted with power factor correction capacitors, which won't do much for harmonic content. Any triplen harmonic content will show up on the neutral; and it is possible that the neutral current exceeds the phase current.
If this is the problem, then upsizing the contactors may simply mean that the next problem seen will be neutral conductor overheating.
A meter used to measure the neutral current should be known to read correctly at 180 and 360 Hz. (Or 150/300 Hz, as appropriate)
-Jon
