Hollow usually means that it is overripe, and risks being bitter.

Tap and hollow sound is what I was taught Guest

Oops! Isn't a marrow squash what we call a zucchini, here?

Zucchini are what we call courgettes, which are immature marrows!

That may be it, then. I guess marrows don't turn bitter when they get big like most zucchinis do, here at least.

Not that I'm ware of and my father used to grow whoppers! Courgettes, zucchinis, I would pick when they are 4 to 6 inches in length max.

When people here want to grow 'em big, I tell them to get seeds of Lebanese squash--like a zucchini, but very pale green, and doesn't turn bitter when it's big.