For sportswear the base size pattern will already contain enough allowances to allow ease of movement during the activity the garment is used for. It is the type of activity and garment type that will impact the design of the grading system. The core consumer of each of these garments are different, the fitted lyrca top is more likely to be consumed by a person concerned with high impact sports compared to a person who is new to sports who would be more likely to consume a regular t-shirt. Therefore you may consider sizing each of these slightly differently to meet the demands of each consumer.
We would also recommend using a single size system (eg. 8, 10, 12, 14, 16) not only because the fitting would be more comfortable but also as this would allow a customer to move down the range as physical fitness increases. This would enable the customer to see a more encouraging change and make them more likely to complete the purchase compared to a dual size system
(eg. S, M, L).