This is non-reproducible with large SLPKs in-house. This is likely not an issue with the size of the SLPK but rather an issue with a specific SLPK or set of SLPKs that violate the i3s specification. Please use the validate scene layer geoprocessing tool to validate the SLPK. If there are errors, please reach out to the vendor who created the SLPK. If there are no errors, please follow up with Esri's technical support and provide the SLPK for further investigation.