compact rsa private key format
In some ways, PuTTY's private key format is more concise as it doesn't include the "reduced" private exponents. BUT, because it includes the OpenSSH formatted public key, it basically has the modulo twice. See this URL for details.

Beyond that I don't think there's any standardized / widely used format that format stores keys in the way that you're proposing. Not MSBLOB, not PKCS1, not PKCS8 (which, for RSA, is basically a wrapper for PKCS1) and not XKMS. If there's a widely used RSA private key format that I missed lmk but of the ones I know about none support the kind of minimalist key you're describing.