Yes, it's fine to do that, particularly when explaining things like this, which is obviously summary data from a scientific experiment. Technical and scientific language strives to be clear and accurate, and is quite happy to be wordy if those goals are accomplished. The explanation of the results is entirely clear, and using different words to avoid monotony would make it less so.
Some corrections: all of your references to animal need to be in the plural: animals. Also, in this particular case I would also use the "serial comma" (the comma preceding and in the final member of a list):
It indicated that small size animals fed preferentially on milk and secondarily on fruits, medium size animals fed preferentially on rice and secondarily on wheat, and large size animals fed preferentially on wheat and secondarily on rice.
Whether or not to use the serial comma is a matter of style: one style manual will say to use it and another will say not to. However, it is always recommended to avoid ambiguity in complex lists, especially where some or all of the items in the last include the word and. Your phrase "preferentially ... and secondarily," which occurs in each item in your list, suggests that the serial comma is to be recommended.