What causes a perfecto leather jacket to flare out at the sides of the hem when unzipped?

[Doctor Death]

This is one of the reasons that I think the cross zip looks better zipped up! Just my opinion.

Also I think going with the smaller, tighter fit should help a lot with this concern. Schott's website has that auto sizing feature that helps you select whether you want a normal fit or a tight fit or a sausage tight fit.

 

[Doctor Death]

another factor is the shoulder slope, if the jacket has sloping shoulder from the neck down to the sleeve post, if you don't have enough trapezius muscle to fill up, then when the jacket worn open the top of the shoulder will rest on your actual flatter shoulder and the rest of the jacket flaring out, you can wear scarf or vest or other mid layer option to fill up your shoulder to stuff the jacket there enough to keep it from flaring out

Or you can pump some iron and fill out those trapezius muscles and deltoid muscles! I recommend Schwarzenegger's book "Arnold: the Education of a Bodybuilder". Fill out that jacket and it'll probably flare out less. You can thank me later.