You can think of a blow-up as whatever happens when air is pushed into something. Air goes in, and the thing swells: a flat sheet of plastic becomes a bed you can sleep on, a small photograph becomes a picture that fills a wall. Keep pushing, and the same swelling turns into bursting: a pipe lets go, a quiet office row finally breaks open in front of everyone. One picture, two endings — bigger, or burst.
From the word’s history
Wiktionary records the noun and the adjective as deverbal from the phrasal verb blow up — blow (send air out) plus up (upward, bigger). The air-into-something picture is therefore already sitting inside the form of the word.
Wiktionary 把這個名詞和形容詞都記為從動詞片語 blow up 來的（deverbal from blow up）。blow 是送氣、吹，up 是往上、變大。也就是說，「氣灌進去、東西變大」這個畫面，本來就在這個字的組成裡。
Wiktionary entry 'blow-up' (etymology_text: Deverbal from blow up; kaikki:3f6f69a190036ae4:849030684 / 849033407)
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