OpenBuildings Designer DGN files can be considerably larger than would be expected for the content, particularly when compared to visually similar MicroStation files.
If the files concerned have been used to create Drawing Views this is generally due to the caching carried out by OpenBuildings Designer where we cache data used to create Drawing Views into the file to improve the graphics display performance of the file. The cached data should result in files opening faster as the Drawing View content can be displayed far more quickly using the cached data.
Unless there are network bottlenecks that impose significant delays when opening larger files, increased file size should not mean reduced performance.
As described in Compress Options in AECOSim Building Designer - Behaviour and Configuration, compressing Building Elements and Display Accelerators will clear this cached information reducing file size.
However, this will slow the graphics performance of the file as the cached data will be recreated when the file is opened, increasing the time taken for the file to be opened and displayed ready for use.
this related content may be relevant: How to accelerate Dynamic Views for large project