Unable to set video capture size

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I'm attempting to set my laptop webcam video capture size to 800 x 600 using:

import cv2
cap = cv2.VideoCapture(0)
cap.set(cv2.CAP_PROP_FRAME_WIDTH, 800)
cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 600)
while 1:
    ret, frame = cap.read()
    width = video_capture.get(cv2.CAP_PROP_FRAME_WIDTH)
    height = video_capture.get(cv2.CAP_PROP_FRAME_HEIGHT)
    print('size:', width, height)

but when this runs, it displays:

size: 640.0 480.0

Why is it capturing at a smaller resolution? Does my webcam only support a maximum of 640x480 resolution or am I mis-using cv2?

I'm running Ubuntu 16, and if I use the program Cheese to test my webcam, it reports a maximum video resolution of 1280 x 720.