This is harder without a side profile picture but it look like your posture is presenting that hollow space in your lower back. You also have a prominent seat. This places limits on the amount you can suppress in the waist in the back of the jacket or you will get what you can see in the above photo which is the start of a fabric "shelf" at your lower back. Doesn't necessarily make the jacket too tight.
Also, the tailor could've taken in thru the armholes to decrease the amount of fabric at the back of the armhole. As a matter of fact, he could've suppressed the jacket by manipulating the side seams in a way that suppressed the back/waist while also decreasing the width of the vents...without touching the center back seam. If you need more suppression beyond that, you can have the tailor add a dart right behind the underarm dart which falls under the arm nearly invisible but helps to suppress the front of the jacket to give you more shape.
Here are some side pictures. I am going to to the tailor soon. What should I tell him? To take in the jacket some more behind the armholes, and what more?