My plastic surgeon says my skin is too tight/thin around tissue expanders to expand anymore. Can a bigger implant be inserted?

I have never personally seen that happen. The premise of tissue expansion is to stretch tight skin.  It may be very thin but that is a different problem

Unfortunately it is unlikely that your surgeon can place an implant in you that has a volume greater than your current tissue expander.  When using tissue expansion/implant reconstruction one of the factors limiting your outcome is the capacity for your skin and underlying muscle layer to expand.  If you are unhappy with the expected size you could undergo a procedure whereby additional tissue is transferred to the reconstructed breasts.  A latissimus dorsi myocutaneous flap is often a good option in this situation.  In this procedure additional skin and underlying muscle is transferred from your back to your front.  Additional tissue expansion can then be achieved allowing for reconstruction of a larger breast.

If
the skin and tissue over the implant are stretched too thin, then you won’t see
the level of coverage that’s likely to give you the best results. I suspect
this is your surgeon’s reasoning behind stopping expansion and using smaller
implants than you would prefer. I would revisit this discussion with your
surgeon for a better understanding of the reasons behind this recommendation.
Remember that the skin, a thin layer of fat and the pectoral muscle
are all that cover your implant - you want as thick a padding as you can
get and overstretched the skin to allow a larger implant may compromise this
padding and increase the visibility of the implant and result is rippling.