There is no problem with it. If you feel that you're writing too much HTML in a single file, consider splitting that bigger file into multiple smaller components. You can also consider the fact that you might be able to re-use some of these smaller components in some other components as well.
Generally, the best way to do this would be to create a folder called ComponentA. Consider splitting the big index.js file inside it into multiple smaller components like dropdown.js, input.js, etc. You can put all these components inside the same folder (ComponentA) or you can put these smaller components in a separate folder so that you can re-use them in some other components.
The other option through while you can reduce the amount of HTML that you're writing in a single file is by using some css frameworks ported for use with ReactJS (react-bootstrap, react-semantic-ui, ant-design, etc). In these type of frameworks, you re-use the components provided by them and just pass props to them. As a result, the amount of HTML in your files should reduce a lot.
Hope this helps!