Word for "underproportionalize"?

If you're going for verbs, it sounds like skew or undercut would point to the inaccuracy of misleading information and the resulting distortion. Another fitting term would be disproportionate.

From your speech,

decreases in expected salary often skew actual decreases in cost of living

Skew v. 2. Make biased or distorted in a way that is regarded as inaccurate, unfair, or misleading. (Lexico)

as cost of living decreases, so does expected salary, but disproportionately so

Disproportionate: Too large or too small in comparison with something else. (Lexico)

as expected salary falls following cost of living, this decreases at a disproportionate rate

In this sense, understate would work just as well in your first sentence, as well as misconstrue, trivialize, underemphasize, and other synonyms for underestimate.