When a creature increases or decreases size, is its movement speed affected? For example, is it normal that a tiny sized scorpion (derived from the large giant scorpion template) has a movement speed of 50 feet? It sounds a bit too much to me.
I don't think it specifically says anything about that in the Bestiary. The 3.5 Monster Manual entry on Monstrous Scorpion had speed 20ft for Tiny, 30ft for Small, 40ft for Medium, and 50ft for Large and larger. Which seems pretty fair to me.
Of course, small critters can move pretty fast.. Just look at wasps, for instance :)