yesterday evening (german timezone here) I was offline, now I can continue.
I guess this is something from the gcc installation, this file is not part of the Mbed installation. When the eclipse configuration is working then it is a great environment, but it is very fragile when there are Mbed updates. Mbed itself is heavily tested, under Linux/Windows it is running very stable now. But it is used by Mac users also, so it must work somehow…
For the first step I would suggest to start with a simple blinky. The warning about the wrong version can usualy be ignored, it may be a problem in very seldom cases. For reproducable results, Mbed is tested with a certain compiler version. But I have used different versions also and did not run into any problem.
The different code sizes will be a result of different compiler versions. A simple test is also to use different compiler settings. Mbed uses sets of settings, called profiles. Default is ‘developer’, but you can use also ‘release’ or ‘debug’. Apply the the switch ‘–profile release’ to the compile command, e.g. ‘mbed compile --profile release’. You will find appropriate directories under ./BUILD. The compiler settings that are used are in ‘./mbed-os/tools/profiles’