As discussed in https://community.esri.com/t5/arcgis-enterprise-portal-questions/portal-for-arcgis-11-3-cannot-creat..., it is not possible to publish webmap bigger than 10MB on Portal for ArcGIS from ArcGIS Pro.
It may seem a lot but it is actually quite easily achievable by publishing a webmap referencing FeatureService from a MapServer (eg: /services/foo/MapServer/X).
As described in details in the Community post (with Python script attached), it is related to way ArcGIS Pro post data. It should be clever enough to switch to 'multipart/form-data' when webmap is bigger than 10MB.
For ESRI people, related case 03981126.
Thanks for listening !
Other related thread:
https://community.esri.com/t5/arcgis-pro-questions/fail-to-save-web-map-in-arcgis-pro-to-update/m-p/...
