Try adding the vla-put-width
(setq txtobj (vlax-invoke space 'addmtext pt 0.0 txt))
(vla-put-attachmentpoint txtobj acMiddleCenter)
(vla-put-insertionpoint txtobj (vlax-3d-point pt))
(vla-put-rotation txtobj ang)
(vla-put-truecolor txtobj tcolor)
(vla-put-width txtobj (cadadr (textbox (entget (vlax-vla-object->ename txtobj)))))
(vla-put-backgroundfill txtobj :vlax-true)
(vla-put-layer txtobj (vla-get-layer obj))