Thank you for the sample database file.
Most of the database space is used by field value in entity class LogVolatileEntityImpl, which contains XML with a very high compression potential. Do you use that field in queries? If not, you may be able to store the content compressed with no need to wait for ObjectDB compression support. You can define a new persistent field, e.g compressedValue, and store the compressed content in that field. The old value field can remain empty in the database, and using JPA events you can initialize value by uncompressing compressedValue on PostLoad and setting compressedValue by compressing value on PrePersist and PreUpdate.
Another improvement can be achieved by replacing the fields context and name (which have repeating values), with enum or references to separate entity objects that wrap theses string (eliminating duplicated content).
ObjectDB - Fast Object Database for Java (JPA/JDO)