Make sure they are in full sun. You are feeding and watering properly. And then wait. If you planted them a year ago, and though they do grow fast... it will still be a few years till they start getting bigger and plusher.

If your trees are sparse as a chronic instead of just being young... it could be they aren't getting enough sun, the food or water is off. It could be a site the tree is unhappy with and it's just not growing well there.

I am also in NJ, Northeast of you in the immediately vicity of Manhattan (was Zone 7a, now Zone 7b). I think it is just a question of time, trees take about three years to establish themselves. First few years of life, trees are establishing ROOTS (all Magnolias are shallow-rooted btw). Magnolias are not in the least difficult trees. Should be NO PROBLEM with water, as it has never stopped raining here! Our soils tend to be moist and acid, so that is a big plus. I may have feed the first few years, but never after that. They will take anything from full day sun to half day. Also, all cultivars are somewhat different, I have Edith Bogues, Victorias and Majestic Beauty and these tend to be very tense, not sure about that cultivar but it listed as one of the cold hardiest, and has great coloration. Good luck.

P.S., Last couple of years, my trees have started to self-sow from seed.

Edith Bogue cultivar of M.g. above

Victoria cultivar of M.g above

They look really gorgeous. Thank you for sharing the pictures. Edith Bogue is very hardy and beautiful too.

So is 'Victoria', and those have reddish leaf buds like Ficus elastica. Majestic Beauty is more common in the South, but have not had issues with it in the North. More than cold, it is SNOW load tolerance that is realy important in NJ. We tend to get get snows with a very HIGH water content, E.B, Victoria and even Majestic Beauty are good with snow loads of heavy, wet snow, and your cultivar is one of the most common in the North.

Dirr and Warren write in their 2019 book 'The Tree Book': [Magnolia grandiflora] "Plants should be tip-pruned when young to develop fuller habits, but left to their DNA as they proceed to maturity. [Although] If properly pruned, a "normal" 80' high cultivar can be aesthetically and functionally maintained at 10' "

I planted 'Majestic Beauty' to hide my neighbors huge, outdoor TV, which is facing my patio. I will be tip-pruning in spring to encourage density and the formation of additional low branches, to better hide the TV.

A huge outdoor tv facing your patio. You have my deepest sympathy, art. That trumps a pool.