I didn't have lobular but my tumour was twice the size than estimated from the mammogra and ultrasound. Our imaging is good, and improving, but the technology is not perfect by any stretch of the imagination. You'll find the increase in size story quite common.
Regarding the stage, like @sister, none of my doctors paid anytime to discussing it. As far as I can tell it's just a short form of describing a patient's diagnosis without detail. Grade and ki67 are more pertinent.
Stage describes the size and spread of a cancer. There are many shades in between so it's further divided, eg Stage 2A. The grade refers to the activity of the cancer cells, how fast they're multiplying. 1 is slow, 3 is fast. It's shorthand for how aggressive the cancer is.
The breast cancer learning curve is vertical. You'll soon get the hang of it, and emotionally it will get better once you have a treatment plan. One day at a time is the way to get through this. Big hug, K xox