I have asked this question myself as well.
I personally think that the landing gear is no issue i the AS350. I am pretty sure they are all the same (B, BA, B2 & B3).
Obviously the B model has a smaller M/R chord which limits its lifting power.
The BA, B2 & B3 have all the same M/R Blades (not 100% sure about B3...).
The B3 has a wider T/R chord which gives it better hover capability at altitude and higher winds with a higher Gross Weight.
I would assume that the MTOW was established at certification to demonstrate its performance envelope and as the AS350 got more power its MTOW has been increased.
It would be interesting to see though how an AS350BA would handle an auto with the same MTOW of an AS350B2 or AS350B3. If they would behave the same, well woudn't that be interesting!!