For murmurs due to stenosis or regurge, increasing blood flow across them should increase the intensity. For example, increasing preload -> increase murmur intensity.
The murmur in HOCM doesn't have to do with a valve abnormality, but with the hypertrophied septum moving into the outflow tract/valve area and producing an obstruction. Since the size of the septum is a static entity, it goes to reason that the larger the chamber size, the less likely an obstruction will be produced, and vice versa. In other words, a thickened septum has a greater chance of creating an obstruction if the size of the LV is smaller.
Hence when we increase preload --> increased LV size due to increase in blood held --> less obstruction --> murmur decreases
When you're thinking about how maneuvers will affect the murmur in HOCM, keep chamber size in mind.
