I don't know anything about BMW 600 engines, but pistons are available in different sizes for nearly all engines.
When new, the engine will have "standard"-sized pistons. When the engine gets old, it may get rebuilt, and as part of that process will have a "rebore": in other words the cylinder is bored out again, to give a fresh surface. The cylinder will now be very slightly greater in diameter, and will need pistons that are also very slightly wider. The difference is a matter of a few thousands of an inch, so pistons might be available in, say "standard", "+20" (for 20 thousandths bigger) "+30", "+40" and so on.
When an engine is rebored, it's usually bored to the minimum extra amount necessary, subject to the new dimension being a recognised size, to fit available pistons. (If you can get away with a "+20" rebore, you do so, so that you might do a second rebore, to say "+40", at some future time. There's a limit to how far each kind of engine can be rebored, but some might allow several rebores - and need several new sizes of piston.)
British engines quote the size difference in thousandths of an inch, whereas continental engines may quote it in metric.
Does that help?