Bigger Nostril Opening Size - Is It Possible?

The number of answers to this question tells you an important fact--there is no easy way to solve this problem. There is no way to replace the missing tissue without additional surgery and scarring, and even then, this is not an easy "fix." Composite grafting or rotation flaps could be attempted, and may well fail due to lack of adequate blood supply to the grafted tissue. If it works, the result may well be larger nostrils, but with substantial additional scarring that could be disfiguring. Thus, the "fix" could be worse than the original cosmetic problem or concern, but at least breathing is improved. You and your surgeon would have to decide if this is "worth it."

One other option would be to consult with a plastic surgeon or facial plastic surgeon experienced in cleft lip/nose repairs who has access to a prosthetics expert, such as at a university or academic hospital. An expandable nostril spreader could be fashioned to wear at night for weeks to months, gradually screwing the device to a larger size and stretching the nostrils. This would act like a tissue expander such as used for breast cancer reconstruction. Two such devices (one per side) could lead to limited improvement over time that would at least preserve the cosmetic improvements you have obtained (at the cost of too-small nostrils).

A 2mm alar reduction is typically does not result in nasal obstruction although other changes made during rhinoplasty can result in nasal obstruction - at 2 months you still have some swelling so I would wait before taking further action

It is very difficult to make the nostrils larger after you had an alar reduction.  You can not simply replace the tissue that was removed.