Sounds like the width measurement included the tires... I found that issue when I was looking at wagons and carts both - in used as well as new directly from the manufacturers. I was so excited by my first wagon - only to find out when it arrived, that it was 2" too wide to fit in my 1969 horse trailer (much narrower than trailers built today) - so couldn't take it to any events! I used it to train my first three pairs (it was too large for any of mine to pull by itself - though I'd purchased it with a set of shafts as well), made some improvements, enjoyed it around my place and got it to one event by having someone else pull my ponies while I borrowed a flat bed trailer to take the wagon. Was a good thing - someone in our driving club fell in love with it and I eventually sold it for what I'd paid for it plus a little more that covered the improvements I'd made. Used that to put the down-payment on the little wagon I have now (AND LOVE!) - which was a whole lot more wagon and cost than the one I'd sold.
It's kinda cute but if it wasn't what you were expecting, I agree, I'd send it back OR you could see if the company you got it from could make updates to it to meet your needs. Or trade it out and go for a larger wagon? It looks sooo tiny - but I have to remember that I have larger "minis" and that while working with pairs I chose to go with a slightly larger wagon (though they can pull it easily).
