Oh you're right you can pack them more discreet, but that should have a worse performance than the one I showed. That's because the coherence length of wireless is fundamentally related to the wavelength -- so to get independent signals you need a certain physical separation, no matter how clever you are with their placing.
Antenna length and placement certainly is a huge factor in wireless performance. But those specific antennas on the linked router are thick and huge mostly because of the plastic decoration around them. As the GP said, it's marketing.
The wavelength is 6cm for 5GHz and 12cm for 2.4GHz, so a 1/2 wave dipole will be half that. (Nobody uses dipoles for WiFi, though. Undesirable radiation pattern, etc.)
