103000 image previews at 150 GB total ~= 1.5 MB per image
At a quick glance that might seem a big large per image, but it will depend on how big your previews are being generated.
See the "catalog settings" dialog, "file handling" tab.
Check your Catalog Settings. If using LR 6/CC 'Standard Preview Size' should be set to Auto(xxxxpx). If using an earlier version it should be set to the horizontal resolution of your monitor or if unavailable the next higher setting (For 1920 monitor set to 2048). Preview Quality 'Medium' is sufficient and will save some space.
With my 2560x1440 monitor the standard preview is 3.7 Megapixels, or 3.7 x 3 = 11.1 MB. With a typical 1/7 JPEG compression ratio (i.e. Medium Quality) the Standard Preview file size will be ~1.6 MB.
Is this normal or am I doing something wrong?
With regards to the size of the preview cache, you can't really do something "wrong", it is what it is.
You do have options, as explained above, to change the size of your preview cache, but you can't say that you are doing something "wrong". The options produce different size previews, not "wrong" or "right" preview size.
Standard preview size "Auto(1920px)"
Preview quality "High"
130,000 images, equals approximately 175gb preview folder size
These figures may be useful to someone as rough guide
I will try reducing preview quality to "Medium"
Thanks for all the replies, much appreciated