I've looked and I've found nothing that indicates that you can allocate more bandwidth to Skype when video calling.
However, you can actually see how much bandwidth you are using with a video call, perhaps compare while closing certain applications that requires the Internet to run/use
Sign in to Skype.
In the Tools menu, select Options...
Under Advanced, select Advanced settings.
Tick Display technical info during calls and click Save.
Important: if you are using Skype 5.2 or above, this function is switched on by default and you can go straight to the next steps below.
In the menu bar, click Call > Call Technical Info.
The Call Technical Info window is displayed.
In the Basic section, the bandwidth used by the video call for both uploading and downloading is displayed.