You upload image with size 598w 900h. You have in functions.php : width'=>1050, 'height'=>900, 'crop'=>false.
This is what happens:
The height constraint 'height'=>900, is ignored, and the width starts getting larger proportionally to the height , keeping the same aspect ratio as the image that it was when you uploaded it until width becomes equal to the constraint in functions.php (1050). It is easy to show mathematically what the height would be proportional to (solve for x) standard proportion equation:
(1050 / x) = (598/900)
(1050 / x) = 0.6645
x = 1050 / 0.6645
x = 1580.14
so your constraint (limit) 1050wx950h created a proportional image (remember crop=false) 1050w/1580h from the initial image size = 598w/900h. Basically 598x900 size image grew while keeping its aspect ratio until it became 1050w 1580h and it would have grown more but your 1050w constraint in functions.php stopped it.
So the question is, what size image should you be uploading that is proportional to your constraints. 1050/900 . * 20% = 210+1050/180+900 = 1260/1080 .... that is the size of the image you should be uploading. if you want to have a nice 1050 by 900 image.
1260 w 1080h ... So please try it out , open Photoshop, make an image o1260x1080 and upload it and see what size slider image you get.
What would happens if you uploaded an image 1400 x 1100 when constraints are 1050x950 and crop = false in functions.php. They are not proportional so 1400 x 1100 will start getting smaller until the *second constraint* is hit. That means that the image will stop becoming smaller *not when the first side becomes equal to the constraint but the second!
So 1400 will keep getting smaller until 1050 stops it OR 1100 will keep getting smaller until 950 stops it. Whichever reaches the constraint in 2nd place will be the width or height identical to the constraint while the other side will be shorter.
I see it in my head but not easy to explain without creating an animation to illustrate the concept.