How can I tell if my MT bike frame is too big for me?

[DoubleJ]

if its working for you then don't change it.

If you want to start riding up steeper hills you may be wanting to consider  a more forward position on the large KTm which means a longer stem

I had a 100mm stem on there, replaced it with a 50mm with riser bars to sit a bit more upright take some pressure off my shoulder, 2 shoulder ops last year and still struggling a bit. Feels better sitting a bit more upright with the shorter stem, the longer stem was just too much pain to handle.

[Captain Fastbastard Mayhem]

I very rarely see tight corners so I'm not phazed about nippy handling...do mostly gravel roads (K2C type stuff) so I cant really say that I need a bike that handles better than what I currently have. 

 

I was just worried that I'm doing my body a disservice in the long run if my frame is too big.

No. And no, it's not too big. It just handles differently to your mate's. It's not causing you any pain or discomfort... 

[Captain Fastbastard Mayhem]

I had a 100mm stem on there, replaced it with a 50mm with riser bars to sit a bit more upright take some pressure off my shoulder, 2 shoulder ops last year and still struggling a bit. Feels better sitting a bit more upright with the shorter stem, the longer stem was just too much pain to handle.

the 100mm would have been a bit long for you, maybe. But if you're comfortable, that's what counts. 

[raptor-22]

I had a 100mm stem on there, replaced it with a 50mm with riser bars to sit a bit more upright take some pressure off my shoulder, 2 shoulder ops last year and still struggling a bit. Feels better sitting a bit more upright with the shorter stem, the longer stem was just too much pain to handle.

 

 

with the 100mm stem you would have needed to slide the saddle further forward on its rails to take some pressure off your shoulders and lower back.  Looking at that frame geo, the 10 would have been too long for the Large frame anyway unless you grew to around 1.64m

[s14phoenix]

I am also 1.78m- ish and ride a large Giant trance. I have also wondered about the frame size issue and if I should rather have bought a medium.

 

Honestly I cannot say. but for me I would rather have a 50mm or shorter stem (on trail bike) on a large than medium with a 60mm+ stem.

[Captain Fastbastard Mayhem]

with the 100mm stem you would have needed to slide the saddle further forward on its rails to take some pressure off your shoulders and lower back.  Looking at that frame geo, the 10 would have been too long for the Large frame anyway unless you grew to around 1.84m

Presume you meant 1.84... 

[DoubleJ]

Presume you meant 1.84... 

I'm turning 40 this year, still I have not let go of the hope that I can squeeze another 6cm in ;-)