Reduced:   Decreased in size, usually from proximal to distal ends, sometimes also changing in shape.   In Example E below (Solidago speciosa), the largest leaf is from the lower stem (proximal), and the smallest leaf is from an upper part of the stem (distal).  Notice that in this example, the shape also gradually changes from elliptical to lanceolate.  This is common in many species of aster and goldenrod.  The progression from large to small may be abrupt (quickly or suddenly) or gradual.
 

In Examples A and B, we could say that the stem leaves are "abruptly" (quickly or suddenly) reduced in size upwards.  In Example C, we could say that the leaves are reduced in size upwards only slightly or gradually, if at all.  In Example D, the leaves are not reduced upwards at all.  In fact, the largest leaves are in mid-stem.