The character for tall 大 shows a person 人 with outstretched arms 一. If you shift the lower strokes towards each other, you can get the: woman 女. The wider lower area can be understood as female hips?
Radical 103 疋 stands for clothing or bales of cloth. In the Hanzi-Trainer, it is used synonymously with the correct 正, because it seems helpful for the explanations and the interpretation clothing/cloth bale would be in conflict with other radicals.
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