1) This is not GnuCash-specific. You can see the same behavior in the gtk-demo app included in gtk2.0-examples.
2) If I SSH into my Bionic box from a computer that's still on 17.10 and run GnuCash on my 17.10 display over the X connection forwarded by SSH, the toolbar are large and have labels.
Two additional notes:
1) This is not GnuCash-specific. You can see the same behavior in the gtk-demo app included in gtk2.0-examples.
2) If I SSH into my Bionic box from a computer that's still on 17.10 and run GnuCash on my 17.10 display over the X connection forwarded by SSH, the toolbar are large and have labels.