if you use OHLC 1 minute backfill (default) each minute is stored as 4 records in raw data, one for each Open, High, Low and Close. It is intended to be used with charts that have frequencies in multiples of 1 minute. If you want sub-minute charts and need to backfill, you need to use TICK backfill which is available with a few different providers. dxFeed, QuoteMedia, others.