What is the DTW result (applied to the MFCCs of an audio file) proportional to?
DTW is not proportional to anything, including frame length. It's not a good idea to compare DTW values between two different pairs like you are doing.

The issue is that DTW is not just the number but also an alignment. If alignment does not match, you can not relate numbers. You only can relate numbers if you align to the same thing.

Normalization is often suggested, but mathematically it doesn't have much sense.

In classical tasks like speech recognition you never compare the DTW values between pairs, instead, you compare a sample to several templates and find the best one. The fact that you try to match to same sample allows you to compare DTW values. In keyword spotting using DTW the threshold must depend on every template you are matching, you can not use single threshold for all the templates. Even more, for the two different templates of the same length DTW thresholds must be different.