It is a reference to the dispute with North Korea/Kim Jong Un, that turned out in the end with close friendship. Where President Thump's big red nuclear button was bigger than his, in reference to the size of the force the US armed forces could unleash.
The tweet is saying to De Santis, the arsenal of negative attention I can unleash on you is greater than what you are capable of, (including 'keyboard warriors', meme reference 'Rob'), we may be at each others throats now, but I be we'll be closer than ever when all is said and done.
https://twitter.com/realdonaldtrump/status/948355557022420992