I don't really have a secret.
That photo was on my phone and I sent it to my email address. I usually get the option to change the size from small, medium, large, or keep it as it is. I didn't get that message this time.
I then received it in my email and saved it as "fish" then I checked the size and it was 154kb which is within the allowable 200kb YT allows and that's how gi-normous it showed up. I usually use Irfanview to resize my photos though and can get them much larger.
If you're using your phone, or your camera, what ever device, you can set the device to what size you want it to take photos. I always have mine set on the maximum because you just never know when you're going to get a nice photo you want to enlarge. That may be some of people's problems. If your phone or camera is set to save at smaller sizes, then there isn't anything you can do to make it bigger. Period. So check your camera settings on your phone. I think that's the secret. Big file=big photo.