Not necessarily. The glass sphere is holding pressure, so it's already under tension. Depending on how thick/strong the glass is, it may already be near breaking tension and not that hard to break. And given that glass under stress wil shatter once cracked, that makes it easier to get out than if it were plastic, which wouldn't shatter. Now, a glass sphere with lower pressure inside than outside would be much tougher, as you would have to overcome the pressure outside as well as the strength of the glass. Kind of like trying to open a car door underwater, before the inside filled with water.