First-weekend demand for the new Apple Watch was much higher than expected, according to analyst Ming-Chi Kuo, who has a reputation for accuracy, particularly when it comes to predicting new products.

The Apple Watch Series 4 has a larger screen and built-in cellular connectivity. It also has an electrocardiogram sensor, which will soon be able to monitor heart activity and can detect abnormalities and health issues, although that feature isn't available at launch. It comes in two sizes, 40 mm and 44 mm, making it much easier to see and use, said CNBC's Technology Product Editor Todd Haselton.

Supply is somewhat constrained by the capacity of the companies building the watches, and Kuo doesn't expect shipments to really pick up until November. He also raised his 2018 shipment forecast from 18 million to 18.5 to 19.5 million units.

The large iPhone XS Max is also outperforming its expectations and is selling over three times better than the XS. The gold and space-gray colors are more popular than the silver.

CNBC's Haselton said that although the XS Max has a larger screen, it's "basically a new model with some improvements" over the iPhone X released last November. It has an OLED display screen, an improved camera and louder speakers than the iPhone X.

The 256GB model is the most popular, and Kuo predicts that sales will keep growing due to high demand in Asia and the upcoming holiday season.

WATCH: CNBC goes hands-on with Apple's new watches