Third-party ISOs can be laced with malware , such as "clippers" that steal cryptocurrency or remote access trojans (RATs).
If you need a smaller legitimate Windows installation for a low-resource or old PC, I’d be glad to write an informational essay on: Highly Compressed Windows 8.1 64 Bit
with the new ESD (rename it to install.wim if needed – some installers require .wim extension). Third-party ISOs can be laced with malware ,
These versions are not created by standard file zipping but through aggressive "debloating" and system modification: Removal of Components: Highly Compressed Windows 8.1 64 Bit
This is built into Windows 8.1 Update 1 and later.