妙法蓮華經玄義 7.203

7.203

餘經通論，約理大妙不殊，而別帶方便；此經不帶方便，故別稱妙。小乘得入，發迹顯本，故別稱妙。

Discussing other sūtras generally, with reference to principle, great and subtle do not differ, yet specifically they carry expedients; this sūtra does not carry expedients, and therefore is specifically called subtle. The Hīnayāna gains entry, disclosing the traces to reveal the original source, and therefore it is specifically called subtle.