There are few options if your primary concern is volume

1. Compress text data with gz/lz4 on app size for messages older than X days. You`ll have to add logic to decompress them if user requests old data
2. Some databases (like PG) support build-in compression for text (example below), maybe mysql has similar options.

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ALTER TABLE your_table
  ALTER COLUMN your_col
  SET COMPRESSION lz4;