Measuring similarity for sets with same cardinality
Yes, the Jaccard similarity score is normalized by the union to deal with sets of different cardinality. Without this normalization (if you used just the intersection), very small sets would always have very low scores. 

When the cardinalities of all your sets are the same, the union of any two sets will be a straightforward function of the intersection (this is easy to visualize - as the two sets intersect more and more, their unions get smaller and smaller). The formula is:

union = 2 * cardinality - intersection


So the Jaccard score in your case would be:

intersection / (200 - intersection)


If you plot this, you'll see it's monotonically the same as what you did.