Complex sentence: Type B plugs in USB 3.0 are somewhat larger; therefore, a USB 3.0 Type B plug cannot enter a USB 2.0 or earlier Type B receptacle.
Split into simpler sentences: Type B plugs in USB 3.0 are somewhat larger than USB 2.0 Type B plugs. A USB 3.0 Type B plug, therefore, cannot enter a USB 2.0 or earlier Type B receptacle.