If its any help, I am one of those annoying "always in the gym" people, I see others come and go as I work out, people of all shapes and sizes, various times of the day.
I have to say, when I see another person in the gym, two things come to mind, firstly, I check out their outfit- hery, I'm being honest here. If its very trendy I normally waste head space trying to work out the label/how much it costed/if I could ever look good in what they are wearing (not normally) if they aren't wearing trendy stuff I tend to not notice them at all.
And the second thing being: are they on the machine I want to workout on next- if so I try to keep my eye on them and feign annoyance if they are on there for a long time!
But weight? body size? unless someone is overly skinny (as in anorexic) I just don't notice. I should add I used to have full blown anorexia (the hospital inpatients..the full works) and this is generally why I tend to notice those that are overly skinny, but no one past that really seems to figure in my mind. Weight or dress size of others...I couldn't even say hi to you if I saw you on the bus, I just don't remember any of the people I've seen in the gym once they have left!
I'd say try to go off peak when the gyms are calmer, even if its one or two day passes, you'd get free personalised fitness advice and also it may correct your myth- just like you don't notice anyone else in the gym once you've left, others also wont notice either, they are too focused on themselves (watching girls in particular once they get closer then 2 meters from a mirror is hilarious!) so please don't worry!
As for the treadmill- I wouldn't fixate on the calorie amount, its not going to be correct as it is just an estimate. Get a heart rate monitor if you want a more correct result and for an even higher calorie burn either try running outside (you burn more calories as your propelling your body forward rather then just up and down as you do on the treadmill) or try interval training: run or sprint as fast as you can for 40-60 seconds (if your not reaching for the stop button after 60 seconds you've not run fast enough) then walk or power walk for 40-60 seconds. It is a better way to burn calories and also increase your fitness.