I agree with sergio2 that your problem maybe isn't to do with the actual capacity [size] of the card itself, but with the amount of data the unit's capable of handling.
All units have limitations as to number of maps tiles and also maps themselves. Numerous small maps with different FIDs will also also trigger a limitation. With an outdoor device you'll probable reach the tiles [segment] limit first as sergio2 stated. While there is not a specific ratio of map tiles to size because of the variation in the sizes of various tiles, it's a fact that it's generally pointless you using a card bigger that 16GB because you will almost certainly exceed the unit's limitations to read all the map data, and your etrex cannot handle more than about 3,000 segments whereas sergio2's montana can handle about 4,000. Auto devices can handle much less again.
That said however, your etrex will maybe even cope with a 128GB SDXC card reformatted from exFAT to FAT32 just as well as a standard 16GB SDHC provided both contain the same data. Did your test involve having exactly the same map data on both the 16 & 32 GB cards? Garmin is now saying this:
Card Size (the amount of storage space on the card)
- 32 gigabytes (GB) - Most Garmin mass storage devices can use cards up to this size
- 64 gigabytes (GB) - VIRB and VIRB Elite
- 4 gigabytes (GB) - Colorado and Oregon 400 series
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