The quick answer is that calcium carbonate contains more calcium per gram than citrate, allowing smaller pills for the same amount of calcium.
Most importantly, we want you to remember to take your calcium and it’s easy to remember when taking your multivitamin and mineral formula, which is always taken with a meal (or immediately following).
Calcium carbonate works best when stomach acid is released with meals, which is not the case with the citrate form. Below is from our Practitioner Dietary Supplement Reference Guide (PDSRG) in case you want complete info with references.
From the Calcium Section PDSRG, pages 1-2
Properly prepared calcium carbonate has been shown to have superior absorption, especially with food.[i] It also has the highest content of calcium by weight (so less is needed) and its cost is less than other forms.[ii] Additionally, calcium carbonate is 40% calcium by weight while calcium citrate is only 21%.[iii],[iv]