Apparently the 5 centavos of that year were 17mm nickel as well. There are two Heritage auctions of a 17mm "small diameter" 10 centavos in which the coins are identified as a pattern that preceded the official issue of 19mm 10 centavos coins. Perhaps yours is one of these patterns or perhaps there was a batch of business strike 17mm 10 centavos prior to the decision to standardize on 19mm. Regardless, an interesting find. Perhaps an expert on the coins of Ecuador will chime in here.
I would certainly recommend taking it to the next major coin show you attend and asking some experts there.
Nice find!