примерно так
let arr = []
arrBig.forEach((elem) => {
let clicks = (typeof arr[elem.user_id] !== "undefined") ? arr[elem.user_id].clicks + elem.clicks : elem.clicks;
let page_views = (typeof arr[elem.user_id] !== "undefined") ? arr[elem.user_id].page_views + elem.page_views : elem.page_views;
let row = { "page_views": page_views, "clicks": clicks }
arr[elem.user_id] = Object.assign((arr[elem.user_id] || {}), row);
})
arrSmall.map((item) => {
return {
id: item.id,
first_name: item.first_name,
page_views: (typeof arr[item.user_id] !== "undefined") ? arr[item.user_id].page_views : 0,
clicks: (typeof arr[item.user_id] !== "undefined") ? arr[item.user_id].page_views : 0,
};
});