This diagram shows the cross section through the vascular system. At the beginning, in the aorta, its cross-section is very small (2.5 cm2). As one more forward towards the capillaries, then the vessels do become narrower, but the number of vessels increases much more. Therefore, the cross-sections of all these vessels (the "sum") increase a lot.
The cross section of all the capillaries together is a total of 5000 cm2, compared to only 2.5 cm2 for the aorta (2000x!).