As some have said, your arms/delts look really good. Although, not to take away from that, but most young men focus primarily on the “see” muscles (the muscles you can see in the mirror). Not to mention, getting these to grow doesn’t involve a ton of effort/weight. Nonetheless, you do have very good development there. What you want to really avoid though is continuing to focus on these muscles while neglecting your weak points. In fact, in the immediate future you should completely shift your focus to your weaknesses so that the divide doesn’t grow larger. Hence, your back and legs. And, as some have noted, getting big legs while being so tall is difficult but can be done. I’d actually recommend you focus more on leg presses and the hack squat machine, with heavy stiff leg deadlifts for your legs. Now, on the leg press, what I found very effective for growth was sets of 20 reps aka high volume! Yes, 20…however, you can, and will need to, do these rest pause to get to 20. So, you don’t back off the weight you use to get 10 reps, but instead you do as many as you can do, then keep your legs locked out while letting the burning subside, then do a few more, rest, then more until you reach 20 reps. Do that for 5 sets, while adding a bit more weight each set, and afterwards your legs will be not only pumped but like rubber! But you’re not done, you need to do some hack squats too, but stick to sets of 10 there. Also, be sure you are changing your foot position on the leg press machine each workout: close and lower target quads, high and spread targets more hams and glutes. You need to hit all areas.
And man, while your upper back/traps/delts look good in your pose, tough love, your lats and mid/lower back are not very developed at all. You need to be doing some heavy deadlifts (or, with your height, rackpulls might be better), barbell/dumbbell rowing, t-bar rows, seated rows, hyperextensions with a weight held to your chest. But for back, you need to go HEAVY but not so heavy that you are just jerking the weight, you need to “feel” your lats engaging. Obviously keep lat pulldowns in there, but vary your grip from wide to close to underhand. I’d advise investing in some lifting straps so that on your heavy sets for back you can wrap the bar, taking your biceps/arms out of it and ensuring only the back is pulling the weights. But I’d advise doing 2 back workouts each week, maybe break it up one for width training, one for thickness. ALso, I’d advise training legs twice a week also, maybe one for quads/inner thighs and one for hams/glutes. But you need to get your focus on hitting your back/legs right now. your arms/shoulders can be trained once per week and youj’ll be fine. Your biceps will still get a workout during back days.