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.
NOTE: With both a text column and an image column in the sample table I'd actually expect 2*59,958=119,916 pages in the estimate, but I'm assuming sp_estspace's calculations assume a single blob column. [Again, I'm not near a dataserver so I can't look at the source code for sp_estspace to know for sure.]
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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)?
NOTE: Keep in mind that sp_estspace accepts a single value (textbin_len) for blob data but you have 2 blob columns (so how much space should sp_estspace allocate to each column?); each blob column also has an in-row lob limit of 2000 (what percentage of blobs should sp_estspace estimate will fit within the data row?); and on top of that both blobs can be compressed (what compression % should sp_estspace use to calculate estimated space usage?); net result is that I wouldn't expect sp_estspace to come up with a very accurate value for total blob page space usage.