I went online to Carhartt to see what size to order.  It said if you wear clothes underneath you should order a larger size.  I am 5'7" 155lbs.  I normally wear 32x32 or 31x32 jeans.  I am not overweight, no gut.  I ordered 36W 32L. These are tight around the waist and stomach!!!  I wear a sweatshirt (non-hoodie) or long sleeve t-shirt under these bibs---after all, these are for winter otherwise I'd order non-insulated bibs.  It's cold outside!!!  Why would I not wear clothes underneath?  Should people go commando while wearing these?
I am a little disappointed in the quality.  Some of the stitching has come loose.  I am not in construction and these are not abused at all.  I wear these for warmth while walking to work and scooping snow.  So, the big question is...would I buy these again knowing what I know now.  I'm not sure.  I probably would buy these again.  They are warm, durable and will last a long time.  But, I would order a 38W 32L.