Your bike is small, light weight, inexpensive and relatively nimble. As mentioned by
@McRider01 the ride with a pillion may have been smoother and felt more planted by addition weight, but did you corner as normal, lean as normal, accelerate as normal? I'm not familiar with your bike, but most likely it doesn't have full adjustment of front and rear suspension, preload, rebound, compression, as other larger more expensive bikes may have. If not, that doesn't mean you cannot adjust the suspension, just that it may take switching out parts as opposed to turning an adjustment.
Generally, when you place more weight on the rear, as in a passenger, the rear end has more sag, that will increase the rake and trail of the front making the bike more stable at slower speeds and in relatively straight lines and mild corner (sweepers), but less agile in tight turns. This may give it a feel of more stability depending on the road you traveled. More weight on the back means it will normally be smoother over road imperfection as well. The difference in weight between you and the passenger can have an even greater effect. Things like tire pressure also come into play.
Heavier, longer wheel base, bigger bikes in general by design make better smoother, stable and planted bikes for cruising, where lighter, shorter, bikes are more nimble for carving canyons. The heavy cruisers feel less difference between solo and 2-up because the additional weight of the passenger is much less of a difference in total gross weight. Lighter bikes there is a much greater felt difference because the weight of the passenger is a larger percentage of the total gross weight.
For every bike and solo rider combination, with all else being equal, there is an ultimate suspension setup and tire pressure depending in the drivers style of riding. Putting 100 or so pounds on the back and all those settings will need to be changed to return it to ultimate. Without changing the suspension or tire pressure what you feel could be better or worse, depending on where you start from as a solo rider.