" But he always wears a medium I don't understand why it doesn't fit him". 95% of the returns to my eBay store are for pet dog clothing that's not the right size for the pup.
I am always willing to work with a customer to find clothing that fits properly and suggest items from inventory that they may not have considered. Every listing in my store has a sizing guide, chart or measurements in the item description. Most measurements I take myself off the actual clothing, but certain brands always run true to size and for those I provide the manufacturer's size chart.
So why aren't pet clothes standardized? Remember the last time you shopped for jeans or pants? The average person (woman) tries on 8 pairs of jeans to find one that fits. What size would you be in a Levi's boot cut? How about St. John's Bay? Liz Claiborne? The pet clothing market is no different. Dog clothing brands are sized completely differently from each other. And that's not counting the Asian sizing of dog clothing (and people clothing) which are several sizes shifted from European or American sizes.
Really important measurements:
To fit your dog or cat properly you need to have several key measurements of your pet. One of these is neck circumference. This is measured around your pets neck with ~ ½" (~ 1 cm) added for comfort. You will also need to know your pets chest measurement. This measurement is taken around the chest at the largest spot which is usually just behind the front legs. Once again add an additional ~ ½" (~ 1 cm) to this measurement for comfort. The last measurement is body length. Measure from the base of the skull to the base of the tail / end of the body. If you do not have a flexible or cloth tape measure you can use string cut to the proper length and compare against a ruler or standard tape measure.
Now you are ready to shop for pet clothing. When comparing and considering an item make sure it will fit your pet best overall. For example, the chest of your pet must fit within the clothing measurements but perhaps the shirt neck would be just a little loose. Or the item is a little short or long. Make the best compromise that will still fit your pet property without being too tight.
But isn't this a lot of work? Yes it is but you will ensure that the garment that you purchase will fit your pet. Who wants disappointment or the hassle of returning the item? Not you and not me. One piece of advice: if you find a brand that fits your particular pet then investigate their other products. I have noticed sizing tends to be better controlled within each brand.
And as I said earlier I will always work with a customer to find the best fitting dog clothes for your pet. I of course want happy customers, but what I love is getting follow-up pictures of your pet looking adorable in great fitting clothes.