logistics guy has a good point. In a 17gal tank, a small betta can make a huge difference, especially the first time you put him in there.
Small uncycled tanks can be considered better than large ones because of the particles in the water that could react with the fish. In a small tank, because of the amount of water, there is a small reaction.
In a big tank, however, because of the huge amount of water, there may be a huge reaction, and production of things like ammonia and nitrate. This may shock your fish.
As long as you are really diligent with your monitoring, it can be done with peace of mind. It has a lot to do with how you prepare your aquarium. You may want to have a small quarantine tank set aside in case your fish doesn't do well in the big tank right away, as well as for any other emergencies you may run into like sickness or injuries.
Aside from that initial risk factor, I'm sure your little guy is going to love that big tank! The live plant will definitely help keep the water levels stable, too.