What causes evolutionary trends?
Science asks not only "what happened?" but also "what caused it to happen?" Upon observing an evolutionary trend, such as a particular lineage increasing in size, a scientist would naturally ask, "What's going on here?" There are many possible explanations.
If we looked at the fossil record for the lineage leading to modern horses, we would see a trend towards increased size. Horse evolution is often depicted as a steady trend, as shown below.
However, when we take note of all the related horse lineages (including extinct lineages), this "trend" begins to blur. Some horse lineages got bigger, some got smaller, some went back and forth. The commonly depicted trend toward increased size is not a general trend at all, but rather it is the result of people limiting their hindsight to the lineage of modern horses.
Functional or physical boundaries
If a lineage starts its evolution close to a limit (such as in size or complexity), we inevitably observe the trend of lineages moving away from that limit.
For example, several times during their evolutionary history, foraminiferans
have been decimated by extinctions, leaving only a few, small-sized
lineages as survivors. These lineages diversified into daughter lineages,
which were larger than their parent lineage. They definitely experienced
a trend towards a bigger average body size, but the trend may have
been based on the fact that foraminiferans cannot get any smaller than
out. They had nowhere to go but up!