In another extremely interesting passage [in the Perfection of Insight in Eight Thousand Lines], Subhūti asks about the meaning of the “great vehicle.”

“What is that great vehicle [upon which a bodhisattva rides]? … Who has set out in it? … Where will it stand?”

The Buddha answers:

“Great vehicle,” that is a synonym of immeasurableness. “Immeasurable” means infinitude. By means of the perfections has a bodhisattva set out in it. From the triple world it will go forth. It has set out to where there is no objective support. It will be a bodhisattva, a great being, who will go forth, but he will not go forth to anywhere. Nor has anyone set out in it. It will not stand anywhere.

The Buddha continues in this vein, but we may skip the text to Subhūti’s answer:

The Lord speaks of the “great vehicle.” Surpassing the world with its gods, men, and asuras, that vehicle will go forth. For it is the same as space, and exceedingly great. As in space, so in this vehicle there is room for immeasurable and incalculable beings. So is this the great vehicle of the bodhisattvas, the great beings. One cannot see its coming or going, and its abiding does not exist. Thus, one cannot get at the beginning of this great vehicle, nor at its end, nor at its middle. But it is self-identical everywhere. Therefore, one speaks of a “great vehicle.”

These ideas are extremely typical of the Prajn͂āpāramitā literature and may be taken as part of the formative matrix in which the chapters on the immeasurability of the Buddha’s life were conceived during the early phase of the development of Mahayana Buddhism. The key point to be learned from these passages is that “immeasurability” is part of a general discourse which seeks to indicate the ineffability of the true nature of things by disrupting conventional terminology. Whatever one can conceive of is part of the world as viewed by discriminating reason.

But the aim, in Mahayana Buddhism, is not to be entrapped by such discriminations. To avoid entrapment, the available terminology has to be used. But it is turned against itself. Thus a very large amount of merit is construed as being so large that it cannot be measured at all. And this in turn points to its “empty” nature, so that we arrive at the realization that a very large amount of merit is so immeasurably large that it is “no merit.”

It is submitted here that such an understanding of the term “immeasurability” underlies the usage in other Mahayana works.
A Buddhist Kaleidoscope; Michael Pye, The Length of Life of the Tathāgata, Page 168-169