When there is a tie in numbering for two substituents, you must use alphabetical order to decide the first-named group. It’s not about size, but the name itself! This always reminds me of the old village register—if two families had the same claim, we would go by family name, not who made the most noise. So, even if the methyl and ethyl both get number two, "ethyl" comes before "methyl" in the name.
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