I'm not so sure that's true of the newer (synthetic) full flow filters on the market. For example, the Amsoil EAO filter I have lists its efficiency as: 98.7% at 15 microns. Now 15 microns is still a lot bigger size than a good bypass will filter down to, but that's also a lot finer filtration than traditional full flow filters did (especially at anywhere near that efficiency level)!
And remember, a full flow filter is so named, because the oil flows (through the filter) at the full rate needed by the engine. OTOH while a bypass filter does finer filtration, it has a much slower flow rate. One effect of this difference in flow rates, is that if some junk in the oil is big enough to be caught by the full flow filter, it's a "race" to see which filter grabs the junk first. And in such a race, the "full flow filter" is more likely to grab that junk, simply because so much more oil flow is going through it! Of course, stuff "too small" for the full flow filter, will of course only be caught by the bypass. However, as the effective micron size of full flow filters improves, more and more stuff is trapped by the full flow (instead of the bypass).
NOTE: One reason I like this effect, is that even the very good full flow filters are cheaper than the (new) price of the OilGuard bypass filters. So if the full flow filters can handle more of the load, than it means I'll have to change the OilGuard units less often.
