Another small cause for a drop in power when using a smaller chainring is greater chain articulation. It’s why track riders often use a huge outer ring and a larger rear sprocket. They get the same effective gear but there is less chain articulation and a small watt saving (single percentages or less). It can be exacerbated by a not perfectly clean drivetrain though. As for pedalling up hills, I did a study on the effect of gradient on muscular activation while pedalling, you can read it here
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