Crossing the BBB depend of many other things besides size of the molecule..

With leronlimab we see most likely interaction with specific transport mechanism present in BBB .
Some type of receptor , or adsorptive mediated transcytosis ..

Many large molecules lack the specific receptors required to trigger these reactions..

We also know of additional factors improving this crossing , like inflammation , and neuroinflammation we see in brain injury and different diseases , like Alz for egz.