Updating Description value for lookup only allows 129 characters in UI but when doing the same via XOG you are able to update it with more then the limit in UI.
DE41416 - Lookup Description Values Update
This behavior was reported as defect DE41416, but after disposition it was determined that it will not be fixed.
It is a known paradigm where sometimes XOG does not have the same rules and the user interface. The XOG is updating data for all languages whereas when the user is entering data within the user interface, it is performing a validation with the minimum length at the time entry. It is expected that the person using XOG has more knowledge in preparing the input files, however for this specific use case, this is a XOG for administration content and typically the data is set up through the user interface, XOG out and XOG into a target environment. Therefore in the typical use case the length of the data value is already pre-validated from the source environment. Therefore, this is something that we will not fix. I would advise the user need to review their input XOG for this use case.