Borrowers applying for adjustable-rate mortgages are asking for much larger loan amounts, raising concerns among housing analysts that ARMs are being used haphazardly—just as they were during the prerecession housing bubble more than a decade ago.

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The average size of an adjustable-rate mortgage increased to $688,400 last week, more than double the average fixed-rate mortgage—which is $280,900—according to the Mortgage Bankers Association. Experts say borrowers are attracted to ARMs for their lower introductory rates, but once they reset to market value after five or seven years, homeowners may no longer be able to afford them.

Last week, the average rate for a five-year ARM was 3.96…