Yes and no.
The more workers you have the more time they have to plant and harvest the food. But you can't get additional food just by having more farmers work on a field. I can't say why you only got 30-50% top. And I never use farms that big.
As far as I know 1 Farmer can handel about 100 tiles² so I try to use 12x8 or 10x10 Fields and that works pretty well for me.
If you use 15x30 Fields you would need arround 5 Workers if my rule is correct. Yields go down because of the harsh conditions. If there is -0°C Temperatur and you have already crops planted the will start to go back again.Or die in Autum.
Also your farmers should live nearby the fields. You can have worker on fields that never work there because the get hungry or cold befor they reach their workplace.

now im up to 11 workers each and the yeilds in some years are good in some crops 70-80% but some years back to 40. if your theory is right then i would use a lot less workers i will try making some 10x10 farms

I recommend 8x12 they fit far better imo