I just bought a house built in 1930s last year and the floors were refinished then. However we did a major remodel in which the floors had to be refinished again. It is now winter in San Francisco and I'm noticing there are major gaps in the hallway in which you can fit a quarter. The rest of the house is fine, there are small gaps around but not as major as the hallway.

What would be causing this? Can wood installed in the 1930s still be contracting and expanding because of the Winter and Summer? The RH in the house right now is around 40-45 with inside temp around 63-66degrees.

Thanks for any input!

Pics
https://goo.gl/photos/gYSePVNd2Uyo99KBA
https://goo.gl/photos/VuG2dfCmjsDoEBQR9

You have a 5/16th X2 inch top nailed floor. A good installation will never have gaps. The main reason I see gaps in a top-nailed floor is because of an unstable sub-floor. When the finish floor runs parallel to a sub-floor that is loosely installed when the house was built there is a chance that the edges of both floors line up.
When the edges line up and the sub-floor planks shrink away from each other it can make a gap.
From the looks of your floor, which has old filler in the gaps between the finish flooring I think that the oak flooring was on the damp side when it was laid allowing the oak floor to shrink some, too, after it dried some. Remember in the thirties it may have been hard to get a loan for a home and the flooring may have sat around for some time in a damp warehouse which would swell it before it was installed in a climate controlled house, which shrunk it back to a smaller width.
A well laid top-nailed floor will be tight after the rows of nails have been installed.
It should be so tight that the edges of the strips press against each other so they move in unison, with no chance of having gaps.
A floor that is above heated rooms below can shrink during the heating season.
Both the sub-floor and the finish floor have shrunk and it is more noticeable
when the rows of flooring line up, one on top of the other in your home.
The only remedy is to add moisture to the air through your forced air furnace to swell the wood back, again, which can cause other wood in your home to swell, too.
A darker finish, makes the gaps not as noticeable. When the floors were finished back in the day, it was common to use linseed oil in the varnish which gives the finish a brown color. I always recommend a "light brown stain" for older architecture. The light modern look that we can get with a "natural finish" may brighten up the house, but shows off any latent defects from the original installation. I say that a light brown floor helps to anchor the architecture.

Thanks Pete for your quick and detailed response! I just didn't expect that these floors would still be expanding and contracting after 80+ years.

Shrink an swell is seasonal and does not matter the age of the floor. I have relatives that live there, been in many homes with the top nailed 5/16 and they all look like that.