Why can scientist be sure about the sizes of many types of dinosaurs but not sure about how they moved or defended themselvs?

In some rare cases the skin or an impression of it is preserved in a fossil. Originally scientists mostly just assumed they had scales. Recently the impressions indicate that some species such as raptors (even the T-Rex) had feathers, probably for mating displays. Others, such as triceratops had bristles indicating that hairy dinosaurs were a possible classification. Both the feathers and hair may have been used for insulation and temperature control.