The design units that are compiled into the working library are kept within a file. This allows for a faster performance since it is faster to load several units from one file rather than loading several units from multiple files. When you recompile the same design, the units that are being overwritten in the library are attached to the end of the file. With each recompilation, more units are being attached to the end of the file which increases the library size.
To avoid this issue, you can compact the library. Compacting the library will reorganize the library file, and the units that were attached to the end of the file will be removed. For example:
alib -lib <library_name> -compact