The “Inspect” tool will build your code in debug mode with no optimizations, to get you a rough estimate of the codesize. See Inspect Memory: is it possible with a release build? - #3 by valeros. This topic has also info on how to analyze a release-optimized binary.
As for the 1.5MB of available flash size: That’s the size of the partition dedicated to the application per the partition file and is never the full flash size. Also nut if you change it with maximum_size – it’s always read from the partition file. See docs.