The AIs seem to think this is not feasible in PDF-XChange Editor. It seems unlikely that this program would lack a viewing feature that Acrobat has had for 20 years. But a representative quote from Perplexity:
For mixed-size PDFs, older Acrobat-style page fitting could feel more adaptive, while many modern viewers—including PDF-XChange—treat zoom as a document/view setting rather than a page-by-page behavior.
The practical workaround is usually to normalize the PDF pages first, or use a viewer that supports per-page automatic fitting in its presentation mode. In PDF-XChange, the closest built-in behavior is still single-page viewing with manual fit, not true page-specific auto-scaling.