What you are seeing is the standard 'sawtooth' pattern of GC. The heap is made up of the nursery and tenured areas. In the nursery the GC performs whats called a scavenge operation
which is a small gc only within the nursery area which will likely account for the smaller jumps you observed in the graph. When you click perform GC it will generate a 'full gc' which
will instigate both a scavenge operation on the nursery area and a mark sweep compact on the tenured area which is why there was a sudden drop in the graph at that time.
GC would have performed that full gc operation as and when it was required and up until then it had not been required and it only happened because you forced a full gc.
It is important to understand that the heap is there to be used so if you see it well used this is a good thing as long as it is performing ok.
