Schubert could churn out large-scale works along with the best of them.
In addition to the Great C Major Symphony, he wrote six compelling large-scale masses, an oratorio, and over a dozen operas (
) ....... all within his short lifetime.
They are not as well known as his small-scale stuff, but he certainly could master large-scale materials ..........
My guess is that he simply never got around to the concerto form/genre .......... 
Or more likely, the concerto was alien to his more introspective temperament. (Which doesn't mean Schubert's works are not difficult to play, at least for pianists - he throws such fatiguing octave passages at the pianist as the ending of the first movement of the Wanderer, and there is no greater punishment for the pianist's right wrist than the accompaniment to Erlkoenig.)
As for subtlety, I don't consider small-scale forms necessarily more subtler than large forms. There's very little in terms of subtlety that can match, for example, Beethoven's treatment of the Neapolitan in the 40-minute-long C# minor quartet, op. 131. But an important point is that, dying at the age of 31, Schubert's output consists in large part of juvenalia. Only towards the end of his short life was he producing expansive instrumental forms like the C major quintet, last three piano sonatas and the unfinished C major (the "Relique"), last four string quartets, and the B minor and C major symphonies. And in this respect he was starting to go in a different direction than that taken by Beethoven. Think of the phantasmagoric middle passage of the slow movement of the late A major piano sonata, or the highly lyrical and expansive opening to the Bb major sonata. (I happen to think that the B minor symphony - as well as the Relique sonata - was left incomplete because Schubert couldn't see his way towards constructing an appropriate finale for either. Certainly the Brian Newbold conclusion for the Unfinished solves nothing.) No way of knowing obviously where he would have gone had he lived as many years (57) as LvB, but what we consider "late" Schubert might well have been relatively early and immature compared to what might have happened.