there are numerous responses to your question you may get from different people seeing your question.
but first - lets get you to qualify - what you define as "tree"????
The term can be deceivingly more complex than you first think
do you mean all your ancestors? all your descendants?
all the siblings of your ancestors? all the wives of the siblings of your ancestors?
all the children of the siblings of your ancestors? all the descendants of all the siblings of all your ancestors?
. . . . the total count can grow and grow and grow - depending on what you decide to include.
to the point that a million people on someone's tree (if we are liberal on what we classify as tree) - is very possible and not an exaggeration.
There are programs like Roots Magic or others that do allow a download from Familysearch - of ancestors, or descendants of person X. But even here - the average person would have to restrict their definition of what they want downloaded - so that it can be done in a feasible/reasonable amount of time and trying to keep the program from being overloaded by immense numbers.