This reminds me of delay line memory, storing information in the echo of a sound: https://en.wikipedia.org/wiki/Delay_line_memory . LEO had long tubes of mercury for this, not sure I would have wanted to work in that office.
I think very few bits here are actually stored in-flight on the copper/fiber/radio at any time. Most of them are probably stored in various buffers of networking equipment (which are ordinary semiconductor RAM). It would be interesting to see someone do an actual breakdown for, say, a transatlantic link. My gut feeling says that only geostationary satellite links have any significant number of bits in-flight.
Very simplified, but assuming a 5000 km cable and 300 000 km/s speed of light (is slightly slower in fiber), light needs 0.01666... seconds for that. At 10 Gbit/s (fairly slow), that'd mean ~ 166 Mbit or ~ 20 Megabyte in flight. (I hope I didn't forget a factor 1000 somewhere)