Why has my Next and Previous buttons gone huge?

Not sure why but in your grey.css drill down to
inherited LI
.post Navigation LI
font-size
Change the size to whatever you want like .4em

Also you have a lot of errors that more than likely contribute to other problems, https://validator.w3.org/check?verbose=1&uri=http%3a%2f%2fwww.originbmxco.com%2f