I know why your hoya is small. It's usually due to a lack of light or putting it in the wrong place that doesn't get light. Try putting it in a place with indirect sunlight or artificial light. The second reason is that your plant may be affected by pests such as mealybugs, scale, and spider mites. Check all parts of your plant for signs of pests. If there are signs, you can immediately give insecticidal soap to repair the affected part.