Just because the garment fits the model loosely or is drawn as loose fitting doesn't mean the garment will be loose fitting on you. Fitting surprises, and I might add disappointments, are the result of this kind of thinking. But do not, I repeat, do not, expect it to fit without some serious alterations.
Most figures have a problem area, be it a large bust, large hips or a large tummy.
A problem area is often the place where clothes are too tight. So it makes sense that before you cut from your new pattern that you go to your closet and measure garments from your wardrobe which do fit in the problem area. For example, if you are making a jacket and you are worried about the new jacket buttoning over your tummy, measure a jacket that fits from center front around the jacket body and back to center front, usually from button to buttonhole. This is accomplished easily by laying the jacket out on a table and measuring the jacket from the inside. If nothing you own fits, put the jacket on and measure the gap in front.