I believe this is very well expressed in Article Four of the Tannisho where the Venerable Master is quoted as saying:

Compassion in the Path of Sages is to take pity on, sympathize with, and care for others. But it is extremely difficult to save others as we wish. Compassion in the Pure Land Path, however, consists of quickly becoming Buddha (as a result of birth in the Pure Land) through the Nembutsu and, with the mind of the Great Compassion and Great Mercy, freely benefitting sentient beings. Because it is difficult to save others as we may wish (without birth in the Pure Land and becoming a Buddha), however, regardless of how much sympathy we may feel towards others in this life, it is not enduring. Reciting the Nembutsu is the only enduring mind of Great Compassion.

Three kinds of compassion are taught in Buddha-dharma:

• Between sentient beings (“small compassion”: shohi)
• Towards those with contact with the dharma (“moderate compassion”: chahi)
• Without reserve to all (“great compassion”: daihi)

The only compassion that we “ignorant beings filled with base passions” can express is “small compassion,” while the compassion of a Buddha is “great compassion.” Some differences of opinion exist regarding “moderate compassion,” but generally they can be said to be the compassion of a shomon , engaku , and bosatsu . As can be determined from the Venerable Master’s words, he considered teaching others and saving them as he wished, to be an extremely difficult matter: “Compassion in the Path of Sages is to take pity on, sympathize with, and care for other. But it is extremely difficult to save others as we wish.”

This may be related to the phrase “those given to wrong doing” in Letter 20 of the Mattoshoalready quoted, or it may be related to the betrayal by his son Zenran who spread a wrong teaching, but because of the deep self-reflection due to his shinjin based on “Buddha-centered power,” the Venerable Master keenly lamented his ability to help others as he wished in the present.

Next is the phrase, “Compassion in the Pure Land Path, however, consists of quickly becoming Buddha (as a result of birth in the Pure Land) through the Nembutsu and, with the mind of the Great Compassion and Great Mercy, freely benefitting sentient beings.”

As indicated in this passage, when we are born in the Pure Land and become Buddha, the “small compassion” of an “ignorant being filled with base passions” changes completely to the “great compassion” of a Buddha, and we are able to save others as we wish.

This is the same thought expressed in Letter 20 of the Mattosho already quoted: “That, however, is not of our own doing. Only when we are saved by Amida’s Vow can we act freely.” And, “Because it is difficult to save others as we wish, however, regardless of how much sympathy we may feel towards others, it is not enduring. Reciting the Nembutsu is the only enduring mind of Great Compassion.”