Hosted agents offer 10 GB of space. You stated that your entire solution folder is 2.6 GB. Your build outputs will typically be somewhere in the range of 2x that size, if not larger, depending on various factors.
If you're a Git user, this the entire repo that's being cloned may be significantly larger than 2.6 GB, as well -- cloning the repo brings down not only the current working copy of the code, but also all of the history.
You can control the clone depth (e.g. how much history is pulled down) by enabling Shallow fetch under the Advanced options of your repo settings.
If you're a TFVC user, you can check your workspace mappings to ensure only relevant source code is being pulled down.
You may be in a situation where the 10 GB simply isn't sufficient for your purposes. If the 2.6 GB is purely code and contains no binary assets (images, PDFs, video files, etc), you may want to start modularizing your application so smaller subsections can be built and independently deployed. If the 2.6 GB contains a lot of binary assets, you'll likely want to separate static content (images, et al) from source code and devise a separate static content deployment process.