Origin of the term "millamolecule"
In their 2013 Annual Report titled Evolving to a Specialty Care Biopharma Company, a well-known pharmaceutical company, Bristol-Myers Squibb has introduced the following statement under the title, Developing Innovative Drug Platforms:


  As R&amp;D evolves its focus, it is also investing in technology platforms that concentrate on new ways to affect disease targets, including antibody drug conjugates, which combine the targeted benefits of biologics with the cancer-killing ability of traditional small-molecule chemotherapies. R&amp;D also will further expand the potential use of millamolecules, which are larger than small molecules but smaller than biologics. These millamolecules may be able to better exploit novel targets and mechanisms by retaining the desirable properties of small molecules with the high degree of selectivity, especially against antigens, that biologics and small molecules have had difficulty targeting.


And then, Bristol-Myers Squibb Company has filled a patent under World Intellectual Property Organization (WIPO) claiming Pet-imaging immunomodulators, abstract of which states that:


  The invention relates to the synthesis and use of $\ce{^{18}F}$-labeled millamolecules for imaging various processes within the body, for detecting the location of molecules associated with disease pathology, and for monitoring disease progression are disclosed.


Thus, to my understanding, millamolecules is the word invented by Bristol-Myers Squibb Company's scientists to describe special set of molecules with different activity other than those exhibited by small molecules and higher polypeptides.

Finally, Medicinal &amp; Pharmaceutical Chemistry glossary &amp; taxonomy describes it as:


  Millamolecules: Mid-range compounds that fall in size between small molecules and biologics. Size is not the only consideration for this class, as millamolecules should also be orally available and able to interrupt protein-protein interactions.


Unsurprisingly, their reference has also directed to Bristol Myers Squibb: Areas of Focus.