I don't have access to a dataserver at the moment but according to the documentation the sp_estspace parameters are:
sp_estspace table_name, no_of_rows, fill_factor, cols_to_max, textbin_len, iosec, page_size
From your example you've provided textbin_len=16 (bytes), which should equate to 1 page for every row's text chain; and since you provided no_of_rows=59958 then I'd expect the estimate to suggest 59958 total pages for the text chains ... and the output shows just that ... 59958 total pages estimated for the text chain data.
At this point it looks like you may have provided sp_estspace with the incorrect input parameters.
What do you get if you run:
sp_estspace 'SAPSR3DB.BC_MSG_LOG', 59958, null, null, 1991830
Does the new estimate/output get closer to your actual space usage (per sp_spaceused)?