The Bernoulli Prionciple explains how varicose veis are more likely to form clots.
When a river is wide, velocity of the water is failrly low.
Similarly, when a river narrows, the velocity increases.
So, when a vein is healthy and normal in size and diameter, blood goes through that vein fairly quickly.
When the vein diameter increases, and it beecomes varicose, blood flow slows down.
Unlike water, slow moving blood has a tendency to form clots.
Hence, a varicose vein has relatively slower moving blood and when the veins get very large, stagnant blood is likely to form a clot. This condition is called a thrombus and when it occurs in a varicose vein, it is called thrombophlebitis.
Injury to the vein from the outside, such as hitting your leg on a car door, injures the inside lining of the vein wall (endothelium) which forms a nidus on which platelets and fibrin deposit, further increasing the likelihood of forming clot as the blood is relatively stagnant.
According to Bernoulli, p1 + (1/2)ρv12 = p2 + (1/2)ρv22
p is pressure; ρ (rho) is density
and the narrowness will therefore increase the velocity to allow the same amount of fluid (water or blood) to flow in the same time it takes at the wider points in the river or vein.
More water or blood has to go through a smaller area so it will need to go faster.