Why is Storyline's font size is different to Microsoft PowerPoint?

Although the slide resolution may be the same, the slide size is probably not. PowerPoint slides are set up in inches, whereas Storyline slides are set up by pixels. If you look at your PPT slide size, it's probably 13.33 inches by 7.5 inches, which is about 1280px by 720px. You'd have to set your Storyline slide size to 1280x720 to really compare the two. If you have a slide that is 16:9 aspect ratio but is set at the Storyline default of 960x540, you don't have a true slide to slide comparison.