Both the chest strap and the girth strap are easily adjustable. Tina will need to do both when she gets the Y strap anyway. I've had to make Gwyim's slightly larger to accommodate the woolly jumper he's wearing. .
I have ordered the next size up as even with adjusting the straps I can't get it to fasten over K9's coat - so Skye, Zeus or Ziva will have have the S size one.
Hopefully the next size up will be ideal, especially as it will have to go over K9's coat - and you have three others that the S size one should fit. The blue really did suit K9 though.