In general, given everything else is the same (same specialty, same location, same employment status, etc...) the more hours you work the more money you should expect to make. Physicians who run their own practices will generally have more flexibility in changing how many patients they want to see and when they want to see them (without having to relocate) than those who are employed. If employed, you may have to relocate and switch to another job to get the hours you want (so it's important to decide how much work vs money you want ahead of time when taking your first job). In private practice, if you limit your hours you would probably want to be careful of choosing times when you'll get the most patients, and if you work too few hours your overhead costs become relatively very high so you'll likely take a hit on your profits.
Many physicians in PP will also work in hospitals because the PP has a contract with a nearby hospital to use their facilities for things that you can't exactly do an office. For example a PP ophthalmologist may see patients in his office on some days of the week for general eye exams but will obviously need to be in a hospital with the proper facilities to do eye surgery on other days of the week. Thus, the physicians are not considered employees of the hospital and don't get any benefits from them (though these PP partners will usually make more than their employed counterparts given everything else is the same since they get to keep all the profits they make without the hospital taking a huge overhead).