एष म आत्मान्तर्हृदयेऽणीयान्व्रीहेर्वा यवाद्वा सर्षपाद्वा श्यामाकाद्वा श्यामाकतण्डुलाद्वैष म आत्मान्तर्हृदये ज्यायान्पृथिव्या ज्यायानन्तरिक्षाज्ज्यायान्दिवो ज्यायानेभ्यो लोकेभ्यः ॥ ३.१४.३ ॥
eṣa ma ātmāntarhṛdaye’ṇīyānvrīhervā yavādvā sarṣapādvā śyāmākādvā śyāmākataṇḍulādvaiṣa ma ātmāntarhṛdaye jyāyānpṛthivyā jyāyānantarikṣājjyāyāndivo jyāyānebhyo lokebhyaḥ || 3.14.3 ||
3. My Self within my heart is smaller than a grain of rice, smaller than a grain of barley, smaller than a mustard seed, smaller than a grain of millet, smaller even than the kernel of a grain of millet. The Self in my heart is larger than the earth, larger than the mid-region, larger than heaven, and larger even than all these worlds.
Eṣaḥ me ātmā, this my Self; antaḥ hṛdaye, inside the heart; aṇīyān, smaller than; vrīheḥ vā, a grain of rice; yavāt vā, or a grain of barley; sarṣapāt vā, or a mustard seed; śyāmākāt vā, or a grain of millet; śyāmākataṇḍulāt vā, or the kernel of a grain of millet; eṣaḥ me ātmā antaḥ hṛdaye, this Self within my heart; jyāyān pṛthivyā, is larger than the earth; jyāyān antarikṣāt, larger than the mid-region; jyāyān divaḥ, larger than heaven; jyāyān ebhyaḥ lokebhyāḥ, larger than all these worlds.
The underlying idea in the verse is that the Self is everything and everywhere. It is therefore smaller than the smallest and bigger than the biggest.