I notice that there are "Big size" helmets and presumably smaller ones, both in 1/35 scale. I don't understand?

German helmets came in sizes. I am not sure how much difference there is between the sizes but they do.

They were of six different sizes:
http://www.german-helmets.com/HELMET%20SHELL%20SIZE%20CHART.htm
Some were available in plastic.

Where did you get the information that some German Helmets were available in plastic?

Brian L. Davis 'German army uniforms and insignia 1933-1945'
According to this book, plastic helmets might be used as part of dress uniforms of aged veterans and wounded/weakened soldiers on various events such as parades etc. They were of the same colour, but not so heavy.

In 1/35 scale the difference in the sizes in the same scale would be minuscule. So,even though they did actually come in a few sizes in real life it is not needed in scale really. They are needed though,because not all 1/35 scale figs are created equal !
The resin helmets are meant to be used with these different sized figs all in the same scale IMHO. There is a big difference between say,a Alpine fig and an older Warrier or Verlindon one.
J