Thick branches should rather be translated clouds. It is a hyperbolical expression in the figure, to express the excellence of the vine of Israel.
Related Commentaries of Ezekiel 19:11
Ezekiel 19:11
11 And she had strong rods for the sceptres of them that bare rule, and her stature was exalted among the thick branches, and she appeared in her height with the multitude of her branches.