The general explanation that is given for this phenomenon is that, in order for tumor cells to grow quickly, they must stimulate the production of blood vessels. VEGF and other growth factors are involved in cancer angiogenesis. Tumor cell aggregates as small as 150–200 μm, start to become dependent on blood supply carried out by neovasculature for their nutritional and oxygen supply.
The prevailing explanation for this phenomenon is that tumor cells need to stimulate the growth of new blood vessels (angiogenesis) in order to grow rapidly.  This process involves factors like VEGF and other growth factors, and even small tumor cell clusters (150-200 μm) become dependent on these newly formed blood vessels for their sustenance.