Absolutely MWS affects career opportunities; however, as a rule of thumb, not absolute rule.
Big MWS = more opportunities but more competition, and typically more relesability later in career for different options/paths.
Smaller MWS = fewer competitors however not necessarily less opportunities, depends how far apart the asset is spread. Positions like staff, test, DETs, etc. vary in size, location, and impact.
For both types of MWS, big amd small fleet: Look at mission, typical TDY locations (i.e. MSN/CC or DET/CC), etc. Opportunities abound in the next few years due to high attrition in the near past. Some briefs now say whomever, Officer-wise, is still in-service in mid-2020's will command whether they like to or not. Don't know if COVID change that on a macro level or not, but do know several (~10) who are staying in for lack of outside opportunities. FGOs will bloat the next few years for this reason surely, and Aircrew Retention TF/CC recently, publicly said this.
LD/HD assets typically experience personnel burnout, attrition, and therefore little releasability when you want it. LD assets have people jump ship for better opportunities on the outside.