Vanishing Twin?

Kelsie • Wife, Momma to 3 girls, expecting baby #4! 🤰
So doctor said I may have a vanishing twin. His ultrasound machine was a temporary one, due to his usual one blowing up lol. So, the top picture is of the "healthy" baby measuring 10 weeks, 168 bpm. The bottom picture is of the questionable vanishing twin, which was measuring 8 weeks, 5 days. Based on LMP, I was 9 weeks 1 day when I went in. He said he may have seen a flickering heart beat for the smaller one. He also said the bigger one was perfectly one, even if the smaller one vanishes. 
Sorry for the long story, but my question is: is the difference in their size a crazy difference, where you'd maybe think the smaller will vanish? Thanks for your input!!