Why are the weight classes so different between boxing and mma.

ex. MMA Lightweight 155. Boxing Lightweight 135.

Is it because MMAers are generally heavier due to training their entire body while boxers mostly train upper. Or is it simply just different sports.

Do MMA fighters also cut weight like boxers or is their weigh ins on the same day of fight?

Lightweight was named when there where no smaller weight classes in pro MMA. Most fighters cut weight usually to the weightclass below there natural weight. Weigh ins are the day before the event usually...

Why are the weight classes so different between boxing and mma.

ex. MMA Lightweight 155. Boxing Lightweight 135.

Is it because MMAers are generally heavier due to training their entire body while boxers mostly train upper. Or is it simply just different sports.

Do MMA fighters also cut weight like boxers or is their weigh ins on the same day of fight?

It's more in tuned to the grappling arts weight class format which has fewer classes then boxing.

And as kaps said, there were a lot of guys at 155 at the time when it was called lightweight.