A different perspective?

Yet those who swear it's size has changed remember it very differently in the past.

One explanation could be straightforward - there are so many places to take photographs of it which contain people, that some may appear to show them closer, whereas others show them further away. Whilst this could happen anywhere, the statue, if viewed from below looking up, would be unusual in that, from several angles, there is no background to frame it's size against, and so using the people on the ground as the reference could create a kind of optical illusion magicians are very familiar with.

Robe differences

Along with the size being different, some people remember the robe not hanging as low as it does today, it not showing the chest the way it does today and there is now a heart showing on the chest which wasn't there previously. Even his chin is said to be different by some.

The claim his robe didn't used to be open is said by the same people who see the heart as new, presumably it was covered up when the robe was closed, as they remember.

Statues changing are not new to the Mandela Effect. The best known one is probably Rodin's "The Thinker", where lots of residue exists in the form of photographs and parodies on the internet. The one of Abraham Lincoln at the Lincoln memorial is quite well known too. It seems completely impossible that a statue can change over time, but those who are claiming this is a Mandela Effect will say it was carved that way originally in a different timeline.