It’s true - smaller and quieter cameras attract less attention.

Yes you’re right that people do notice you putting up a camera to your eye even if it’s a smartphone. When photographers say a camera is “invisible” I don’t think they were referring to its literal meaning they really mean people feeling more comfortable and thus not caring.

Do people see your smartphone when you’re taking their pictures? Yes. Do they care? No.

Same with smaller cameras - do they see you? Yes. Do they care? No.

With DSLRs or any big, fat camera it usually goes like this - do they see you? Yes. Do they care? Oh boy do they throw a paranoid fit before calling security and threatening a lawsuit.

Having said that, a big part of stealth is your own body language. Some photographers are so sincere, upfront and comfortable with whatever they’re doing that people feel at ease among them even when they’re having their picture taken up close with a big DSLR. On the other hand, some photographers can have the smallest stealthiest cameras yet behave like sneaks so that’s obviously going to attract a lot of suspicion (people can sense insincerity and fear).