Having had a quick look at wikipedia (not the best source, I know, but the writer is drawing van der Kiste's book: Kings of the Hellenes, which is reliable as far as I know), I think it quite possible that Sophie simply didn't have much jewellery in her possession at the time of the wedding.
According to wiki, the day after Alexander's coronation in June 1917, the rest of the Greek royal family fled from Athens to the small part of Otropos. Since on the evening before the crowds in Athens had refused to allow them to go even as far as Tatoi, I rather think that they could not have managed to carry much with them. The earliest time Sophie could possibly have returned to Greece was when Constantine was restored as king on 19 December 1920 (none of the family, apart from Queen Olga, were allowed back before that, not even to visit Alexander on his deathbed - and even Olga got there too late). I haven't managed to establish whether Sophie went to the wedding in Bucharest from Athens or from exile - if the latter, it's hardly surprising that she didn't give her daughter-in-law very much.
The fortunes of the Greek royal family remained chaotic for the rest of Sophie's life. She and Constantine were exiled again in September 1923. Constantine died four months later and Sophie remained in exile until her death.
If Marie of Romania and her daughter were complaining about the size of wedding presents against all this background, it strikes me as pretty small-minded.
Ann