I don't think inverters that are too large are necessarily less efficient, if you mean converting DC to AC. They tend to be less cost effective if you are paying for inverter output capacity that is unnecessary or underutilized.
Aside from soiling and degradation, the other reason that nameplate DC to AC ratios can be above 1 is simply that real world conditions usually underperform the test conditions (STC) that module nameplates are based on.
When the OP says 1% clipping he is presumably talking about estimated annual energy lost due to the inverter limiting output. It is not a direct reference to the DC-to-AC ratio. I suspect that our OP is getting a 1% clipping figure from software I suggested in another thread. Regardless, to see 1% clipping it probably means his DC to AC ratio is already meaningfully above 1, probably at least 1.05 if not 1.1. So as you add more PV to the inverter, you increase the energy production but also the clipping percentage, and you get diminishing returns from each additional module added. At some point it may be more effective to increase the size of the inverter to boost energy production than to continue adding more modules, but it all depends on the prices for everything involved and is not a simple calculation. For residential systems it isn't worth the time to do such calculations with precision, just make sure the customer understands your choices, or give them a larger inverter if you cannot reason with them. (Usually if they need to pay for a service panel upgrade for the larger inverter then they understand your reasoning.) For MW scale systems it's worth having someone spend weeks estimating the right ratio given whatever prices and constraints there are.