Concrete5 default installation size right after install a fresh copy is about 31MB. Many developers prefer to have a lighter version of projects, for so many reasons. My reason for make every installation lighter is to make commiting the source and also backup/restore files faster.
This list contains files and folders you can remove from your project safely because using of them is so rare and in special conditions, but before any action, make sure you will not need them in the future. And i just list files that have considerable size, not files that removing them does not effect on size too much. You can decrease overall size about 32% by removing these files:

1- Secureimage (used for captcha) audio files, 72 .mp3 and .wav files with 1MB size. Delete them if you do not have any plans for using sound captcha.
location: /concrete/libraries/3rdparty/securimage/audio/

2- Zend Local data files, there are 476 .xml files with over 8MB size.
General files that are needed are:
en.xml, en_US.xml, en_GB.xml, metazoneInfo.xml, root.xml, supplementalData.xml, telephoneCodeData.xml,Translation.php.
And keep 2 xml files for every language/country you are using, e.g. for French keep fr.xml and fr_FR.xml. Rest of the files...you remove them.
locationt: /concrete/libraries/3rdparty/Zend/Locale/Data/

3- Adodb other than mysql driver files, 53 files with 0.5MB size. Delete them if you do not have plans for connecting to any database other than mysql.
Keep adodb-mysql.inc.php, adodb-mysqli.inc.php, adodb-mysqlpo.inc.php , adodb-mysqlt.inc.php, adodb-pdo_mysql.inc.php.
And you can delete the rest of files at this folder.
location : /concrete/libraries/3rdparty/adodb/drivers/

4- Adodb doc files, 11 files, 620KB
Location: /concrete/libraries/3rdparty/adodb/docs/

5- JSON library document files, 23 files, 134KB
Location: /concrete/libraries/3rdparty/JSON/doc/

Also if you want to continue, there are lots of files, especially language and doc files that you can find and remove from your project folder.