My husband is about 170 lbs and he wears a 34" waist pant although I'm not actually sure of his waist measurement. When he was down to about 155lbs he still wore the same pants but they were a little loose and he needed a belt.
I can tell you that all of my pants fit whether I weigh 150lbs or 165lbs and I don't feel much of a difference in how they fit no matter what I weigh (wish I did then I would know when I put on a few pounds!). I can also tell you that when I go from 165lbs to 150lbs I only lose between one and two inches in my waist.
You go ahead and use that "just washed them" excuse as it happens to be true. I hate when I wear a pair of pants and they feel so comfortable then you wash them and they are tight in places and you feel like you have gained ten lbs. I have had days when I swear I must have gained 5lbs only to get on the scale and find I haven't gained a thing. I always put on the fresh washed pants and do a few squats and lunges to stretch the legs, then I put my thumbs in the waist and pull out to stretch that
It also shouldn't matter if we think you are the "right" size. If you think you need to work on yourself then by all means, do it. If not then don't. I also don't think you should worry about the sizes of the clothes you wear. I can tell you that I have clothes that range from size 10-18 and they all fit but not all size tens in the store fit me (most don't!) and not all 18's fit me either. Not all clothing companies have the same sizing standard. I wish they did because I wouldn't have to try on 20 pairs of jeans only to walk away empty handed.
Everyone is built different. Every shirt I buy is either too big through the chest or too small everywhere else. Almost every pair of pants I own are too tight in the thighs and hanging in the waist. I am an average of a size 14 through the chest and a 10-12 for the rest of my top. I am a 12 waist and a 16 thigh and somewhere between petite and regular for the length of my pants. I could probably lose 30 lbs and still not fit into anything properly. Do I need to lose 30 lbs...it probably wouldn't hurt but I judge my body by the way I feel in it. I can honestly say that at 160 and up I feel uncomfortable in my skin. My fat and skin bunches up and pinches me, I can't sit or lay down comfortably and it just really annoys me because I am always aware of it, not because I see it but because I feel it. If I weigh under 160lbs I feel (literally) much better in my body because nothing is pinching or bunching!
Sorry for the long post! I think you should be proud of the fact that your tight, tight pants now fit you! Now you will need moderate exercise and a nutritional diet to keep them that way (easier said than done). Good luck to you!