That's easy to explain: there's a liquid in the bottles pushing back against the glass walls. It's likely that as the bottles sank, the higher water pressure from the ocean could have leaked into the bottles. This would equalize the pressure.
sealed with air still inside.
Pressure is measured in area kind of, pounds per square inch. A significantly smaller container doesn't go under nearly the same amount of stress as a larger one. I'd also be willing to bet if you compare cross sections, the smaller one would have more of its volume in the walls vs the submarine.
Hummmm...
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