Understanding Society User Support

Hi,
I'm running analyses using the above variable in Understanding Society (US). However, I am seeing discrepancies in the distribution of employer size in US data as opposed to employer provided sources such as the Business Population Estimates (BEIS). It seems that US data reports a higher proportion of individuals in the smaller companies (those with less than 50 employees). It could be that individuals who work for branches of larger companies report the number of employees in those specific branches rather than the company as a whole. I wonder if this is true based on previous experience of researchers using this variable? Is there any linked datasets with US that I could access that would provide the employer provided number of employees in the company? Any pointers would be much appreciated.