By your picture appearance they definitely look oversized you need a mock up fone prior to any cosmetic work . Will require corrections or replacement

Altering of the veneers chair side will surely affect the strength of the veneers.. also it will result in cracks in veneers leading to staining and dislodgement of veneers in near future . Kindly visit your dentist and express your concern regarding this .. and possible get it replaced with new ones .

Hi... Was the a try in given before hand? U may need to get them repeated as doing any changes in them will affect it's strength n longevity.

Dear practo user You need to get them repeated

It is very difficult to comment on the thickness of the current veneers.. For that, we need to see before pics too, along with models and case study.. But apart from that, in general, thickness of the veneers depends upon the correction required.. Here it does seem that proper smile designing was not achieved and cosmetic results are sub-par.. The thickness of these veneers per se can't be reduced.. If you are unhappy with them, then you will have to get it completely redone..