You're working on a performance issue and you are trying to find the fastest way to process an enormous table. Wouldn't it be time for a bit of archiving ? How "big" are the tables on the system anyway ? Check out transaction DB02, which holds a lot of very relevant information on high level. In Mega / Giga / Terra or Petabytes.
When you are looking up the size a tables consumes on the system, know this: indexes on the table are likely to be around and they too are space consumers.
The transaction reveals a lot of detail on the system's table sizes and occupied space. On the main screen it shows what percentage of the system's capacity is being used. It's not real-time information you are looking at, it will be as recent as the last "collector run", which is probably not much more than a few hours up to a day.
Mind you - if the DB02 transaction is a bit too high level for your table investigations, try the DB02OLD transaction.
Top 50 largest tables
To view these, simply select Space / Segments / Overview and then tab Top sizes. The tables are presented by size in Mb where indexes of the table are included in the total size. Be aware of this: tables which have large fields don't actually occupy all that space. The database is clever that way. Ergo: occupied table size and number of records in the table are only vaguely related.
Index effect
Where a table with slow performance can be "indexed", which is a very effective performance improver, the down side to using indexes is table space. An index is in itself a table which means key fields and indexed fields are stored twice. For some tables a single index is not enough. In some cases, the combined size of indexes on a table exceeds the size of the table. For the systems largest tables, which are also widely used tables (coincidentally ?), the combined index size adds up to about a quarter of the space needed for the table. Thats quite considerable!
Each table has it's key, which is in fact itself an index. Thus transparant tables always have an index, index ~0.
Monitor growth
So how fast is the system growing? Run the DB02 transaction on day 1 and again on day 21. A small calculation will reveal how many Mb the system grows every day. Alternatively check out Space / Database / Overview and then tab History. This will show an overview of the percentage of space that is used by the system, in a day-by-day overview. Or weekly, or monthly, whichever you prefer. The system size in Gb and freely available space in Gb is also shown.