EV 12.5 doesn't create index for thousand files - Items missing content. The setting of parameter "Maximum conversion size" is 100MB. I checked some suspected files where size of each file is perhaps 5MB. There are *.xls files. I have read this KB - www.veritas.com/support/en_US/article.100042766.
So, I am not understand this description "Note that if you increase the value, and the conversion output exceeds the default size but is less than the set size, then the item might not be indexed at all; not even the attributes"
It seems that if conversion size of the file is larger then original size of xls file, the indexing of the content the file is stopped.
to be honest I am note sure what exactly they mean with this statement but I can tell you that we've seen instances especially with .xlsx and .zip files (compressed file types) where items were not properly converted not due to the "Maximum conversion size" setting but due to the conversion timeout settings in the site. EV is supposed to report problems with content conversion in the "Veritas Enterprise Vault Converters" Eventlog. So if you do have the saveset id of a item in question you can try to locate an entry for this item in this log that might give you more details about what went wrong.
Two addition articles in regards to that: