I have never played any of the Final Fantasy games and have never really been fond of that manner of game play.
Additionally, the Tifa character just never held much interest to me and she has never seemed to remarkable to me from a physical standpoint. She has always looked like a typical video game female from the mid-90's.
So - I did some digging into Tifa and the Lulu characters mentioned above and this is what I discovered.
Tifa Lockhart originally appeared in FF7 back in 1997. Her physical attributes were an item of note from the start and it should be noted here that in most of the information at the time the discussions appear to center around her overall bust size but not so much the appropriateness or lack of any proportion as was the case in later discussions.
Part of this may have to do with the little mentioned similarity of Tifa (FF7 1997) to Lara Croft (Tomb Raider 1996). Not only do the have many similar clothing elements but they have actually moved toward each other in physical stats.
Originally Tifa was listed at 5'-4" and has become standardized at 5'-5". Lara started out at 1.8m (5'-10 3/4") and is now canonically listed at 5'-6". Their busts actually started out exactly the same size in terms of bra - 36DD. Both have vacillated form that point, but they could have swapped tops with not a problem.
I did a great deal of digging into original confirmed design information and yes, Tifa and Lara were 36DD in their first appearances.
Lulu from Final Fantasy 10 actually became a very interesting review process and I gained a good deal of insight into how a video game character's look actually comes to be. To cut to the main points, Lulu is said to stand 5'-6" tall and after a great deal of research it turns out that under her extensive and camouflaging clothing she sports the equivalent of a 38H bra.
This is not just idle fan speculation but comes from the actual 3D mesh model used in the game. It turns out that Lulu's model is actually a scaled up Tifa mesh framework. Her decolletage and overall clothing were all intentional. Meticulous attention was paid to not only the look but the implications and symbolism of her attire. Much like Lara, Lulu's final chest size actually was the partial byproduct of a mistake. To have Lulu's chest render correctly and look "correct" from the various camera angles the simple scale up from Tifa was not enough and she was given a boost of about 2 cups. This in turn caused some concern and the fur line wrap was extended to hide the true profile of her chest.
I did find some fan made mesh renders images showing Lulu in less attire and they actually do show a figure in keeping with a 38H figure. Superimposing these less clothed renders over the base model show they have not changed the bust - at least that portion visible in the fully dressed model, again lending credence to the reported mesh adjustments.
Lulu also was intended to have a more mature and maternal figure due to the plot needs. Her chest was therefor intended to be fuller and heavier in a nurturing manner.
So, in conclusion, Lulu is significantly larger than Tifa.
Tifa is 36E/DD and Lulu is 38H.