Log backup size in full and bulk logged recovery model will be roughly the same size.
Full recovery the entire size of the data inserted goes into the log records and hence gets backed up, so it's data + log headers + some overhead. Bulk logged the modifed extents are included in the log backup whereas the amount logged is small, so the log backup has data + log records + some overhead. The sizes will vary, usually by small amounts. The log backup in bulk logged recovery is not going to be twice the size of the one in full or anything close to that.
The tests Tony and I did writing the tran log book had the DB in full recovery model having a 1GB log and a 1GB log backup, the database in bulk logged having a 24MB log and a 1 GB log backup.
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: Discussions on DB performance with occasional diversions into recoverability
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