What does it say on your tickets?
When we flew with AirIndia in November we traveled in & out of the international terminal

Air India I think are the only ones that di that as some of their Mumbai to Goa flights originate in Dubai.

Just booked our flights for November
Virgin LHR>BOM
Air India BOM>GOI
Noticed that on the Air India you now only get 15kg baggage
I emailled them and was told that extra baggage is charged at 250inr per kg.
You do get 46kg if you book both sectors (all the way through) with them.
We booked direct with AI for less than £180 return for 3 seats.
Tickets To India wanted £199 for 2 and you still only get 15kg
So if you factor an extra 10kg works out at £25 its still a lots cheaper

BA changed my Jet flight from Mumbai to Goa to an Air India one because the Jet Flight changed to a Jet Connect and they said they couldn't book it...personally think they made a bit of a slip up but not really a big problem. However when BA called me to discuss our options they said that Air India would only be 15kgs not the 23 we have with BA but when I've checked the booking it's showing as 20kgs so you may be the same.
We travel fairly light so I doubt we would use all our allowance anyway.