I understand that Steph is 6-3, 185 and Monta is 6-3, 180. I understand that there are some larger (though not necessarily quicker) backcourts that in a perfect world would be better defended by larger and bulkier guards, particularly at SG. I understand that a 6-6 superstar SG like Kobe will present problems for someone of Monta's stature (but who doesn't have problems with Kobe, of course). I understand that it was great to have Baron at the point, because he could guard bigger, bulkier SGs and allow Monta to take on the smaller, slighter PGs. I understand that both Steph and Monta's defensive credentials were suspect prior to this season.
What I don't understand is everyone (including Monta) ignoring the actual data about backcourts in the NBA, and how a Steph/Monta combination, while on the smaller side, is not unusually so. And while not directly supportable by data, I think it is an easy argument to make that a Steph/Monta combination presents matchup challenges the other way, to larger backcourts lacking the speed and explosiveness of Steph/Monta.
Of the 29 starting backcourts in the NBA other than the Warriors (quasi-starting today, i.e. adjusting for injuries and other issues), fully 17 of those (59%) have a SG that is 6-5 or smaller, and all of those are without large, bulky bodies that would prevent Monta from guarding the ones nearer 6-5 (think Ray Allen). Of those 17, 9 of them (or 33% of the league including the Warriors) send out backcourts with a SG that is 6-3 or smaller, and none of those is particularly bulky. So a Steph/Monta backcourt is very similar to a sizable portion of the league today.
And why might one-third of the league be able to play with smallish backcourts? Because, per the above, almost two-thirds of the league has a SG 6-5 or smaller, and of modest builds. Furthermore, of the remaining 12 teams with a SG over 6-5, only 3 of those have the speed and athletic gifts to be truly difficult matchups, and they are not surprisingly the ones that have superstars, or wannabe-superstars (Kobe Bryant, Brandon Roy, and Joe Johnson). Also, 3 of the 12 are not difficult matchups physically either owing to an even more modest physical build (Rip Hamilton), limited skills (Sefolosha), or age (Carter). There could be a debate around the remaining 6, but all have limitations of some form that make the physical matchup far less intimidating than it otherwise might appear on the surface (Ariza, Brewer, Brewer, DeRozan, Jackson, Martin). And lastly, Monta has shown flashes of arguably brilliant defensive play (see vs. Brandon Roy earlier this year) and Steph Curry has improved markedly all season.
So there seems to be nothing unusual, or impractical about a Steph/Monta backcourt from a defensive standpoint,assuming Steph continues to improve and Monta can have some of his offensive load removed to allow some defensive effort from him as well. And the offensive explosiveness of Steph distributing at the point and Monta playing the far more natural 2 guard for him, can be something to behold.