This is a known issue, which has been mentioned several times in the forums already. The browser version of the viewer is the old pre v3 release, this is because of the reported issues. This includes out of memory and huge experiences of lag. As the old viewer was more stable we decided to roll it back for web.
This had one drawback and that was the liveries would appear thin. I suggest until the web viewer is updated with fixes, if your car is too small, you can change to an icon or initials so you at least know where the car is.
I must stress this is just a UI bug nothing in your cars performance is affected.
I am working on the problem now it is just taking a while to debug the reasons it could happen, I feel I am close but I don't want to commit to a time as yet.