I noticed the apple guidelines mentions that app icons are 120x120 but when I go to something like photoshop or fireworks to see how that looks like, it looks much bigger than that. Anyone know why?
It probably has to do with the pixel density on the iPhone or other smartphone screens. iPhone 6 Plus has 401 pixels per inch, it is probably 4 times the density on your computer monitor. In other words, 120 pixels will be a tad over an inch on your computer monitor but will be about 0.3" on the iPhone display.