I think you mean to say "discrete transistors" (transistors that are sold per piece) instead of "analog transistors" and "integrated circuit (IC) transistors" instead of "digital transistors."
Transistor size is determined by power requirement. The higher the power requirement, larger will be the size. In case of discrete transistors, they are built to a high specification of power because they will be used for multiple applications. Whereas, the power requirements in IC transistors is more or less known, so they can be built to have minimum size. Another reason discrete transistors are big in size is practical handling of transistors. How will you handle a 100 nm sized transistor? How will you even see it? If you can't see it easily, how will you place it on your breadboard or PCB? So, they are made big enough to be handled with ease.
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