The most appropriate synonym for "magnify" is "expand." "Magnify" means to make something appear larger or more important, and "expand" aligns with the idea of increasing in size or significance. The other options, such as "magnanimous" (generous), "miniature" (smaller version), and "underemphasize" (reduce importance), have different meanings and do not capture the meaning of enlarging or making something appear larger.

An Achilles' heel (or Achilles heel) is a weakness in spite of overall strength, which can lead to downfall. While the mythological origin refers to a physical vulnerability, idiomatic references to other attributes or qualities that can lead to downfall are common.