"MOSAIC" is a kind of painting or you can say a decoration usually filled with small pieces of stone, glass etc.
I here compared a "mosaic" with "life"
I feel like "I am also a mosaic, big mosaic" here "mosaic" means life, which must be filled with "lot of different pieces." "Different pieces" here represents happiness and sorrow, pleasure and pain, enjoyment and stress, love and hatred, struggle and hard work and failures and disappointments sometimes, because living isn't easy at all.
And "have lot of empty spaces" here means unpredictable future or empty future which has to be filled as well. We don't know what future holds for us. So, there are still many more things in life we have to go through and we don't know of. We have to witness many things we don't know they actually exist.
And once the "mosaic", which contains the "different pieces", are gathered together and placed on their suitable or "empty spaces", creats a life.