Nexplanon - Is it normal that my arm where the insertion is fatter then the other one?

I googled this one and the reviews were virtually universally bad with the exception of a (presumably paid) blog article that basically said it was the best thing since free air. The biggest issues seem to be improper implantation and migration (the girl had skinny arms, so of course it would have been easy). I read of one case the same as yours (search results) which turned out to be a bogus link to the 100% positive blog. The 'unbiased' blogger claimed a 99.95 success rate (better than tubal ligation plus a condom).
While they admitted that extended bleeding could be a problem they claimed that only 91% had the implant removed because of it.
Just do your reading and try to find reputable sources or you can end up reading total rubbish.
See your doctor about this I think.