Objection 1. It would seem that more angels sinned than stood firm. For, as the Philosopher says (Ethic. ii, 6): "Evil is in many, but good is in few."
Objection 2. Further, justice and sin are to be found in the same way in men and in angels. But there are more wicked men to be found than good; according to Ecclesiastes 1:15: "The number of fools is infinite." Therefore for the same reason it is so with the angels.
Objection 3. Further, the angels are distinguished according to persons and orders. Therefore if more angelic persons stood firm, it would appear that those who sinned were not from all the orders.
On the contrary, It is said (2 Kings 6:16): "There are more with us than with them": which is expounded of the good angels who are with us to aid us, and the wicked spirits who are our foes.
I answer that, More angels stood firm than sinned. Because sin is contrary to the natural inclination; while that which is against the natural order happens with less frequency; for nature procures its effects either always, or more often than not.
Reply to Objection 1. The Philosopher is speaking with regard to men, in whom evil comes to pass from seeking after sensible pleasures, which are known to most men, and from forsaking the good dictated by reason, which good is known to the few. In the angels there is only an intellectual nature; hence the argument does not hold.
And from this we have the answer to the second difficulty.
Reply to Objection 3. According to those who hold that the chief devil belonged to the lower order of the angels, who are set over earthly affairs, it is evident that some of every order did not fall, but only those of the lowest order. According to those who maintain that the chief devil was of the highest order, it is probable that some fell of every order; just as men are taken up into every order to supply for the angelic ruin. In this view the liberty of free-will is more established; which in every degree of creature can be turned to evil. In the Sacred Scripture, however, the names of some orders, as of Seraphim and Thrones, are not attributed to demons; since they are derived from the ardor of love and from God's indwelling, which are not consistent with mortal sin. Yet the names of Cherubim, Powers, and Principalities are attributed to them; because these names are derived from knowledge and from power, which can be common to both good and bad.
Source: Summa Theologica (New Advent)