What does the last phrase 'when a sphere' mean?

Question

link to the book At the Mountains of Madness

I encountered this sentence and can only guess what it means, but I want to know for sure

They were normally shapeless entities composed of a viscous jelly which looked like an agglutination of bubbles; and each averaged about fifteen feet in diameter when a sphere. They had, however, a constantly shifting shape and volume; throwing out temporary developments or forming apparent organs of sight, hearing, and speech in imitation of their masters, either spontaneously or according to suggestion.

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Question

Does the phrase 'when a sphere' - when you deal with a sphere mean? If not - I appreciate the right answer then

Answer 1

You can understand it to mean "when it took/had the shape of a sphere" Compare: He practiced many arts, poetry, painting, music, theater, but most people would say he did his best work when a painter. In other words, "when [being] a painter".

The amorphous entity was constantly changing its shape. Every now and then it would assume the approximate shape of a sphere. At such moments, its diameter was about fifteen feet.

The UFO was constantly changing shape. Sometimes the UFO was round like a hamburger, and sometimes it was elongated like a sausage. When a hamburger, its diameter was about fifty feet.