What sin is opposed to magnanimity by defect?
It is the sin of pusillanimity (CXXXIII.).
Why is pusillanimity a sin?
Because it is contrary to the natural law which inclines every being to act according to its capabilities (CXXXIII. 1).
It is then indeed blameworthy not to make use of the powers and the means God has given us by the mistrust of oneself, or by taking up an unseemly attitude with regard to honours and glory?
Yes, this is indeed blameworthy and should not be confounded with true humility, about which we shall speak later (ibid.).
Next - The Catechism of the Summa - XLIV. OF MAGNIFICENCE; AND OF THE SINS THERETO OPPOSED, VIZ., OF STINGINESS AND OF EXTRAVAGANCE