Open it in HxD and see how much blank data it has.
Cause maybe they used 512MB cartridges for that game but it actually only uses up less than 200MB of it (which would be weird cause they could just have used 256MB cartridges instead.)
next time check it in the database if you're unsure if it's the right size: http://3dsdb.com
for that game, the trimmed size (which will basically be the same size in CIA format) is 181407744 bytes which is roughly the 173Mb that you have there.
Some 3DS to CIA converters also have a tool to give you the info about the rom, and one of those infos is precisely the unpadded size of the rom.