How do you tell winter coats are too bulky for car seat?

I own the gracomyride 6 and rear face my 17 month old dd. Since it is winter, bulky coats are dangerous and can cause coat compression in an accident. So how can you tell the coat is too bulky? I put my dd is a bodysuit of fleece, put her in the seat and tighten the straps very tight. Took her out, and i was able to somewhat pinch the collar and stuff two hands on top of eachother under her straps of her belly. Is this still too loose? I looked somewhere that you cannot put more than two fingers under the strap of the childs shoulder and I am not able too.