When using the font modifier, set a weight to the font you're passing.

For example, if you want to use one of the default text styles (which I recommend, since they adapt to the user's Dynamic Type setting), you can do it like this:

Image(systemName: "arrow.right")  .font(Font.title.weight(.ultraLight))

If you want to specify a font size, you can do it like this:

Image(systemName: "arrow.right")  .font(Font.system(size: 60, weight: .ultraLight))

For UIKit, symbols can be configured as follows:

UIImage(systemName: "arrow.right",        withConfiguration: UIImage.SymbolConfiguration(pointSize: 16, weight: .bold))

SwiftUI 1.0

I just wanted to also mention how to change the weight along with a custom font size.

HStack(spacing: 40) {    Image(systemName: "moon.zzz")        .font(Font.system(size: 60, weight: .ultraLight))    Image(systemName: "moon.zzz")        .font(Font.system(size: 60, weight: .light))    Image(systemName: "moon.zzz")        .font(Font.system(size: 60, weight: .regular))    Image(systemName: "moon.zzz")        .font(Font.system(size: 60, weight: .bold))}