strap extension

I don't think you're reading what Apple suggests about the extension correctly. If you buy the extension band, it lengthens the 49mm Ocean Band by an additional 50 millimeters. There is only one extension size you can purchase. It is primarily designed for people who dive and need to be able to strap the Ultra watch over a wetsuit.

Lay both sides of the original band you bought together (end to end) and measure the length. Then lay the one end of the band which doesn't change with the extender (end to end) and measure the length. The difference should be 50mm longer. You should only be replacing the side of the band with no hardware and still use the side of the band with the buckle, which came with the original band.

I don't understand how that is possible. Here is what Apple says specifically about the extension:

Made from the same high-performance elastomer as the Ocean Band, this band extension lengthens the 49mm Ocean Band by an additional 50 millimeters — perfect if you need extra length to wear the band over your wetsuit.

The extension comes with an extra adjustable, corrosion-resistant titanium loop which you can place in any tube to tuck the excess band into. This extension must be used with the regular Ocean Band.

At this point, I recommend you reach out to Apple directly for help.

I meant to add that the standard Ocean Band will fit wrists from 130mm - 200mm. If you buy the extension that means it will fit wrists from 130mm - 250mm.

Post a photo of the extension strap, next to the one which came with your watch. You should ONLY be replacing the original one side of the watch band with the replacement extension piece.