I don't know the density of PETN and I'd prefer not to fire up a VPN to run that search at the moment, but let's assume it's something like 1.5 g/cm³

So 20 grams of the stuff would occupy 13.3 cm³

Which is equivalent to the volume of about three and a half AAA batteries.

How big are these pagers, and how can there be that much empty space in one?

Specs:

- Dimensions (mm): 73(L) x 50(W) x 27(H) (98.55 cm^3)

- Weight: 95g including battery

Presumably the majority of that weight is battery. And 13.3 cm^3 would occupy less than 1/6 the total available volume.

Battery is described as "Lithium battery, up to 85 days with 2.5 hours for full battery charge, USB-C charging."

From: <https://www.gapollo.com.tw/product/ar-924/>

Archive: <https://archive.is/Kw0Pg>

(For some reason the origin is unreachable presently.)

From comments and news stories elsewhere, it seems highly probable that the explosive was engineered into a battery form-factor, and replaced the original battery of the device.