Exchange online and Hotmail are two different products. You can not use Office 365 subscription to improve the functionality of your mailbox, it will create a new mailbox for you.
Based on my searching, you could buy outlook-personal-email-plans to expand your Outlook/Hotmail mailbox. But, I'm not sure if it can expand the allowed attachment size, you could confirm from the Outlook.com forum. The forum is support for Exchange related question.
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